Added support for multiple slaves to pgsql.

When you have a single postgresql cluster managed by corosync
and pacemaker you cannot have any further slaves either with
streaming or logical replication. This patch enables connecting
other postgresql servers as slaves to the cluster without breaking
or interfering with the running cluster.

By default the new option is disabled to prevent broken setups when
upgrading from older versions of the script.

Signed-off-by: Dimitar Ianakiev <dimitar.q@siteground.com>