Add new option pg_restore_command.

This can be used so that repmgr standby clone adds the string
specified in repmgr.conf as a restore_command in recovery.conf.

We can use this option for integration with barman by setting the
parameter to an appropriate get-wal call.

repmgr.conf.sample

@@ -113,6 +113,10 @@
 #
 # tablespace_mapping=/path/to/original/tablespace=/path/to/new/tablespace
 
+# You can specify a restore_command to be used in the recovery.conf that
+# will be placed in the cloned standby
+#
+# pg_resore_command = cp /path/to/archived/wals/%f %p
 
 # Failover settings (repmgrd)
 # ---------------------------