StackStorm Packs - Support for Git Submodules

[jk464]

StackStorm Packs are implemented as Git Repos. Currently when pack install is run, submodules contained within packs are not handled.

This PR implements the checkout_submodules option on pack install (and pack download etc) so that when pack install is ran a user can optionally set this option to True to checkout submodules as part of the install process.

The option defaults to False to maintain current behaviour of the pack install process and thus backwards complatibity with pre-exisiting processes.

Implements #5814

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[guzzijones]

please sign the cla

[guzzijones]

oh looks like this just needs a review and a milestone