ci: strip GIT_BRANCH
ci: strip GIT_BRANCH

GIT_BRANCH is typically 'origin/master', we strip the variable to only
get 'master'

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

JrCs/ceph-docker,ceph/ceph-docker,JrCs/ceph-docker,JrCs/ceph-docker,ceph/ceph-docker