Add Git driver

This adds a very basic git driver, essentially so that Zuul can
use it to load its configuration from a non-subject system.  This
facilitates third-party testing (where the configuration is under
the control of the zuul operator in a git repo, but the subject
system is otherwise unrelated).

Change-Id: I26829dbcbeeb23fec3a7978767071d9d28ea5597
10 files changed