Update pack install action so it works correctly when --python3 flag and "python_versions" pack.yaml attribute is used

[Kami]

This pull request updates / fixes packs.install action (and st2-pack-install binary) so it works correctly when pack.yaml metadata file includes python_versions attribute and when --python3 flag is used for installation.

Background, Context

In v3.0.0, new python_versions attribute was added to pack.yaml. With this attribute pack can declare which Python versions it supports / works with (Python 2.x and Python 3.x). This attribute is used by our Exchange CI to run checks and tests under correct Python versions.

In addition to that, it's also used by packs install action and binary. If user tries to install a pack on an unsupported Python version, an error is returned.

In the past, we added --python3 flag to pack install command. With this command, user can use Python 3 binary for a specific pack when StackStorm itself runs under Python 2. This pull request fixes this functionality so it works correctly when pack metadata specifies python_versions attribute.

NOTE: Mixing Python 2 and 3 has a lot of edge cases so I also plan to document that (a lot of packs may not work, especially the ones with rely on C extensions, etc.). Once Ubuntu Bionic packages with native Python 3 support go GA we should probably deprecate --python3 flag or at least make it less prominent.

Resolves #4654.

[Kami]

I'm re-running Ubuntu Xenial tests. I have no idea why they failed 2 times in a row.

I verified it locally under Python 3 and everything looks in order.