No vip in shared-db binding space should cause unit to block

We have found an issue where a percona-cluster charm relating to vault was setting the db_host IP for mysql to the unit's IP rather than the VIP configured and in use by the database cluster.

Looking at the code here, the logic notes that if the VIP is in the current relation's interface's CIDR, then the VIP will be configured in the shared-db relation. Otherwise, the unit's private IP on that network space will be set.

https://github.com/openstack/charm-percona-cluster/blob/master/hooks/percona_hooks.py#L734

I believe that with the majority of connections being made to percona-cluster charm via the shared-db relation, and all of those relations, in a clustered percona-cluster environment, should point to the VIP, we should block the charm (or otherwise inform the operator) if no VIP is a member of the shared-db interface binding network CIDR.