Replace virl.pull.cli.do_pull with Git Pull using GitPython

[tybruno]

Problem

Currently, virl.pull.cli.do_pull uses requests to do a git pull. It also uses a static url instead of allowing users to enter their own repo.

Fix

Refactor the code to use GitPython and allow users to pass in their own repo url's for their own use case.

[tybruno]

I am will be fixing this issue w/ tests to verify it works and does not cause refactoring errors then submit a PR.

[tybruno]

@andrewschenck

[xorrkaz]

I'm down with some refactoring here. And when you say "static repo" you mean something other than GitHub. The repo itself is flexible, but the repo "hub" is not.