Allow configuring permanent replication slots in patroni

Patroni handles replication slots automatically when new nodes are added to the "cluster". This does not work when you are adding a standby cluster, as it lives in a different DCS. The only way to enable a persistent replication slot in that case is to manually create them or by defining a configuration variable in Patroni to create and keep that alive.

That configuration variable needs to be exposed in Omnibus.