In some resource-constrained environments, particularly during service
bootstrap Galera cluster nodes can experience timeouts in inter-node
communication.
This change sets the gmcast.peer_timeout based on the galera cluster
documentation:
https://galeracluster.com/library/documentation/galera-parameters.html
We are observing peer timeout issues on some CI runs - therefore raising
it to PT15S as in similar Ubuntu charms jobs.
Change-Id: Id036e41b62a88bab486c35a5f1fde5cfc2fa4803