Allow to configure git base URL

Makes the devstack-gate usable with a generic git repo.

git.openstack.org is hard-coded in functions.sh. Make it
configurable via GIT_BASE. Set defaults in functions.sh
just in case there is a consumer of functions.sh other than
devstack-gate-gate-wrap.sh.

Define GIT_BASE in devstack-vm-gate-wrap.sh which
defaults to git.openstack.org, so that it can be set
via JJB changes only (layout.yaml).

The name of the "trunk" branch where devstack-gate is
fecthed from, and where grenade gets the latest code from,
is hard-coded to "master.

Define GIT_BRANCH in devstack-vm-gate-wrap.sh which
defaults to master.

Change-Id: I59c1c63d6e8ac8482821728bf03bfce69898e919
Merge "Allow to configure git base URL"