Create a git cache for tripleo-gate.

This is a little redundant with prepare_devstack, but since we want to
migrate to /opt/git soon anyhow (so we aren't chasing new
repositories) I think this is fine. We also update prepare_devstack so
that it won't go off and cache the tripleo repos during test execution
time.

Change-Id: Id9706cf824b0b75fe11787a2612e75aa79c3507f
Co-Authored-By: Robert Collins <rbtcollins@hp.com>