Check for Ubuntu 16.04 and conditionally install Python 3.6 and configure ST2 to use it

[blag]

The end-to-end tests are failing on Ubuntu 16 because they don't have Python 3.6 available, which is blocking my PR #5064:

not ok 36 packs.setup_virtualenv with python3 flag works
# (in test file cli/test_pack_python3.bats, line 88)
#   `RESULT=$(st2 run examples.python_runner_print_python_version -j)' failed with status 127
not ok 37 python3 imports work correctly
# (from function `assert_success' in file cli/../test_helpers/bats-assert/src/assert.bash, line 114,
#  in test file cli/test_pack_python3.bats, line 117)
#   `assert_success' failed with status 127
# 
# -- command failed --
# status : 1
# output (19 lines):
#   
#   {
#       \"action\": {
#           \"ref\": \"python3_test.test_stdlib_import\"
#       }, 
#       \"context\": {
#           \"user\": \"st2admin\"
#       }, 
#       \"end_timestamp\": \"2020-10-29T00:47:52.928925Z\", 
#       \"id\": \"5f9a1138f016f26a2a042272\", 
#       \"result\": {
#           \"exit_code\": 1, 
#           \"result\": \"None\", 
#           \"stderr\": \"Traceback (most recent call last):\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/netaddr/compat.py\\\", line 91, in <module>\
    from importlib import resources as _importlib_resources\
ImportError: cannot import name 'resources'\
\
During handling of the above exception, another exception occurred:\
\
Traceback (most recent call last):\
  File \\\"/opt/stackstorm/st2/local/lib/python2.7/site-packages/python_runner/python_action_wrapper.py\\\", line 53, in <module>\
    from st2common import log as logging\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/st2common/log.py\\\", line 32, in <module>\
    from st2common.logging.handlers import FormatNamedFileHandler\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/st2common/logging/handlers.py\\\", line 22, in <module>\
    from oslo_config import cfg\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/oslo_config/cfg.py\\\", line 336, in <module>\
    from oslo_config import types\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/oslo_config/types.py\\\", line 21, in <module>\
    import netaddr\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/netaddr/__init__.py\\\", line 18, in <module>\
    from netaddr.core import (AddrConversionError, AddrFormatError,\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/netaddr/core.py\\\", line 11, in <module>\
    from netaddr.compat import _callable, _iter_dict_keys\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/netaddr/compat.py\\\", line 93, in <module>\
    import importlib_resources as _importlib_resources\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/importlib_resources/__init__.py\\\", line 31, in <module>\
    from importlib_resources._py3 import (\
  File \\\"/opt/stackstorm/st2/lib/python2.7/site-packages/importlib_resources/_py3.py\\\", line 20, in <module>\
    Resource = Union[str, os.PathLike]\
AttributeError: module 'os' has no attribute 'PathLike'\
\", 
#           \"stdout\": \"\"
#       }, 
#       \"start_timestamp\": \"2020-10-29T00:47:52.507461Z\", 
#       \"status\": \"failed\"
#   }
# --
# 

This PR tweaks the st2_pkg_e2e_test workflow to check for Ubuntu 16 and if detected, installs Python 3.6 from the deadsnakes PPA and configures StackStorm to use it utilizing the patch utility and Bash heredocs.

[blag]

This code does not change what StackStorm itself runs under, and yes, on Ubuntu 16 and RHEL 7, StackStorm itself runs with Python 2.7.

What this change does is tweak what version of Python StackStorm uses when installing packs with the --python3 flag (st2 pack install --python3 mypack). Previously it was using the system's Python 3 version, which is Python 3.5 for Ubuntu 16. Unfortunately, Python 3.5 does not implement os.PathLike, so we need to tell StackStorm to use Python 3.6+ when installing Python 3 packs.

One point to consider is that this should work just fine for the current unstable builds, but we might want to consider continuing to use Python 3.5 for the stable builds on Ubuntu 16 and any 3.3.x releases.

[arm4b]

Per discussion we agreed to provide python3.6 PPA via curl|bash installer based on user's y/n input. See: StackStorm/st2-packages#681 and we can adjust this PR afterwards.

That would be better as adding repo beforehand defeats the purpose of end-to-end tests.

[blag]

I reworked this to use echo 'y' | when running the bootstrap script.

[amanda11]

Excellent - LGTM - should fix the E2Es.

[blag]

Once our current E2E builds finish up I'll install this branch on st2cicd and try it out. I won't merge this until it's at least tested.

[arm4b]

That should work in this particular case.

However I provided --u16-add-insecure-py3-ppa flag as identified more bash corner cases with the interactive mode under different environments. StackStorm/st2-packages#682. It might be more clear to rely on that new flag.

[arm4b]

Feel free to pass that U16 flag to script directly without extra workflow logic as our installer is catch-all and won't fail in case of non-existent CLI param provided.

[blag]

Even better!

[arm4b]

As StackStorm/st2-packages#682 is now merged, you can add --u16-add-insecure-py3-ppa