Allow overriding of OpenStack release

This also uses a friendly default debian branch name of
debian/<release codename> if the debian branch cannot be
found from the tag name. This makes it easier to build
source projects that don't follow the OpenStack release
tagging convention.

Depends on https://review.rc.nectar.org.au/913

Change-Id: I0c4da010626f9543ff97f9759455e1a3a680c379