text: Title: Have st2 pack install checkout git submodules when cloning a pack repo
  Body: When a Pack is installed on Stackstorm using `pack install` - stackstorm performs a clone on the Repo ([Code](https://github.com/StackStorm/st2/blob/dfab25aab0468f330a635db55f01286523217135/st2common/st2common/util/pack_management.py#L205)) - however this results in any submodules present in the repo not being checked out and only being an empty directory. 
  
  It would be useful if the `clone_repo` function also checked out any submodules that are present in the repo, as it would be helpful to add dependencies maintained in seperate repos to the pack as a submodule, while still being maintained externally.
  
  In my use case a series of ansible playbooks are maintained in a seperate repo and I would like to also make them available as a pack to st2, and as I see it the best way to do that is via git submodules.