[nova] Moving nova-cells-v1 job to the experimental queue

I have changes up which move the nova-cells-v1 CI job to the experimental queue: https://review.openstack.org/#/q/topic:bug/1807407 We continue to see intermittent failures in that job and given the deprecated nature of cells v1, CERN is using cells v2, the job no longer runs nova-network, and its flakiness (plus the general state of the gate lately), it is time to move this further out of our daily lives and down the road to its eventual removal. Keeping it as an option to run on-demand in the experimental queue at least allows us to leverage it if needed should some major cells v1 fix be needed, but that is doubtful (we haven't had a cells v1 specific bug fix in a long time). -- Thanks, Matt