Add new --python3 flag to st2 pack install CLI command

[Kami]

This pull request adds new --python3 flag to `st2 pack install`` command.

When this flag is provided, virtual environment which is created for a pack uses python3 binary (based on the value of actionrunner.python3_binary config option).

Underneath, it adds python3 parameter to packs.{install,setup_virtualenv} actions.

This pull request builds on top of @tonybaloney's idea in #3922.

We couldn't come up to a consensus in that PR. I thought that approach in that PR is still a bit too invasive until we have CI, etc. in place for packs so we can verify end to end that pack also (or only) works with Python 3.

This pull request goes with a more incremental middle of the ground opt-in approach. If user wishes to use Python 3 for a particular pack they can opt-in. And if things don't go well (stuff doesn't work under Python 3), they can easily recreate virtualenv / reinstall pack with default Python 2 binary.

In the future, we can also build on top of this approach and perhaps utilize pack.yaml metadata attribute to signal if pack additionally / only works with Python 3 as suggested in #3922.

TODO

• Tests

[bigmstone]

I'd rather python3 be python_version, but it's not a blocker. Meaning I'd rather this code be able to handle future versions of python too with minimal changes...but I don't know the likelihood of that being a thing.

[Kami]

@bigmstone Yeah that might change in the future when whole StackStorm platform supports and runs under Python 3 :)

[Kami]

@tonybaloney Hopefully this PR will work as a good compromise until we have the whole platform to run on Python 3 :)

[tonybaloney]

👍