Make the debug facility a "normal" gitdm option

Allow the environment to influence DEBUG, and rename it GITDM_DEBUG to be more unique

Change-Id: I4ea078435d2ddcd1fb9177887d70c1d3f861ad13
Reviewed-on: https://review.openstack.org/31665
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins