Also add configuration file option "pgdata" for hard-coding the
node's data directory - if the "repmgr" DB user isn't a superuser
or doesn't have permission to extract the data directory, we'll
need another way of finding out.
This is needed for better switchover control, so we can instruct
the remote repmgr to issue the appropriate server command rather
than trying to work out what it should be from the local node.
