add mergify as a merge engine
add mergify as a merge engine

From now on, each PR will be merged automatically if:

* there is no DNM label on the PR AND
* the PR has at least one approuval AND
* the travis CI successfully passed

Closes: https://github.com/ceph/ceph-csi/issues/154
Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi