If "archive_cleanup_command" is defined in "repmgr.conf", a corresponding
entry will be made in the node's "recovery.conf" file after cloning a
standby.
Note that we recommend using PgBarman to manage WAL archives, but are
providing this facility to help repmgr to be integrated in existing environments.
Implements GitHub #416.
get_superuser_connection() was erroneously using the local node record
to connect to as a superuser, which works when registering the primary
but obviously not when cloning a standby.
Addresses GitHub #380.
