Enable yardstick-docker-build-push-gambia job

Enable yardstick-docker-build-push-gambia job

It replaces the yardstick-docker-build-push-fraser job, because the
logic in jjb/releng/opnfv-docker.sh L106-L119 has limited options for
docker image tags:
- 'latest', for git commits on master branch
- tag-name, for git tags
- 'stable' otherwise, i.e. for git commits on *any* stable branch.

This means we can have only one (stable) job building and pushing
docker images, in order to prevent the 'stable'-tagged image from
switching between fraser and gambia contents, depending on the stable
branch where the most recent change was cherry-picked and committed.

Unfortunately, disabling the yardstick-docker-build-push-fraser job
also means that further git tags on the stable/fraser branch will not
result anymore in building and pushing new docker images with the
intended tag-name and contents.

Change-Id: Ieec1cdc6230e4c000c159cd743c587561c788fc3
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>