"Incomplete relation" errors on cinder, heat, gnocchi, aodh if percona-cluster is 'late' in the bundle in terms of deployment

Essentially, what seems to be happening is that if percona-cluster is not 'early' in the deployment, then cinder, heat, gnocchi, and aodh (at least in one test run) don't complete their shared-db relations as percona-cluster never fills out the relation data with username/password/etc. (it may also not setup the database; I didn't check that).

The scenario is the kitchen sink:

 * percona-cluster *3 with hacluster
 * just about every service there is.

This is on xenial-queens (testing as part of the series upgrade).

Removing the shared-db relation and re-adding it causes the the relation to complete (assuming that percona-cluster is fully set-up).

I suspect what is happening is that percona-cluster is still in the process of forming a cluster and thus can't handle the relation-joined from the associated units. Therefore, it defers, but never (once it has formed the cluster) gets around to checking if it had to defer clients on the shared-db relation. There probably needs to be a check at the end of clustering to see if it needs to go around and setup some databases for clients that were deferred.

I'll also test the same bundle without a cluster of percona-cluster (i.e. in single db mode) and see if it exhibits the same behaviour.