If the deployer decides to override the complete apt repository
git, it would be best to have the possibility to NOT define a
filename, this way it would be the same behavior as in N.
This also allows a combined backport of
https://review.openstack.org/#/c/417362/
to fix bug 1653947 cleanly in Newton.
Change-Id: I00b924275cd9350a93b48f4d886919b597392049