Don't clone from git unless variable is true

This particularly useful for Vagrant machines. This is because we are
likely to have the code directory set up as a shared folder.
Thus we want to do everything in git_application, apart from cloning the
repository.
Only set up known_hosts if pull_from_git is True
Merge pull request #1 from EDITD/only-clone-from-git-if-told-to

Don't clone from git unless variable is true