[cern] Allow overwriting labels on swarm mode creation
Allow passing label values on cluster creation for swarm mode. This is available in all kubernetes drivers as well as swarm, but somehow missed on swarm mode.
Story: 2004942 Task: 29343
Cherry-pick: https://review.openstack.org/#/c/634501/
Change-Id: Ie3ac66f45e27cc92993116c3df0b33873dc67e24
Conflicts: magnum/tests/unit/drivers/test_template_definition.py