Add integration test for multihost connection strings in Postgres using replication manager

[robmonte]

Postgres supports multiple hosts in a connection string like so: connection_url="postgresql://{{username}}:{{password}}@hostone:5432,hosttwo:5432/postgres"

With pgx, Vault now supports this too. To have test coverage of this feature, we can use postgres containers with replication manager set up in a cluster and then test generating credentials on it.

Only the leader can perform writes and the secondary instance is read-only. By performing a shutdown on the leader, we can test that after a failover event occurs the secrets engine still works as expected.