Cluster upgrade may have trouble with single-quoted CLUSTER_UUID

Scott points out (I think it's what he means) that the postinst script for maas-cluster-controller, when migrating an old-style CLUSTER_UUID setting in the celery config into maas_cluster.conf, doesn't cope well with single-quoted uuids even though that is actually what we use. The regex was meant to support that, but the language isn't exactly easy reading.

The potential problem was not actually observed in manual testing of the given scenario. We may have switched from double quotes to single quotes at some point, in which case the fix also needs to be backported.