Increase flexibility of stackrc repo config.

 * Offline use of devstack previously required defining individual
   repo overrides.  This change maintains support for individual
   overrides while adding the ability to override GIT_BASE.

Change-Id: I4ca8b8e69c0fb05a7c9cf2fd4643eac2f7643aa9
Merge branch '25362-git-base' into dt-test

Conflicts:
	stackrc

Change-Id: I1600c834af7b15af9ccd9620aced3a3b474727e5