Make tools/integration.sh take a branch
Make tools/integration.sh take a branch

Instead of always using master branch support the option to use stable
branches. Read the branch via the $OVERRIDE_ZUUL_BRANCH environmental
variable that devstack-gate sets

Change-Id: Iab4ae1efddc4dca38dcd3779dae1a79ddc2b0852

openstack-dev/pbr,JioCloud/pbr,redhat-openstack/pbr,openstack-dev/pbr,klmitch/pbr,redhat-openstack/pbr,WiFast/pbr,varunarya10/pbr,varunarya10/pbr,klmitch/pbr,openstack-dev/pbr,WiFast/pbr,JioCloud/pbr