Add --ignore-rsync-warning command line option

This fix the rsync return code in case there are vanished files.

Common situation are DROPed tables and TEMPorary object deletion and
are handled by PostgreSQL.
But as it may exist situation where an external process delete files in
the PGDATA the flag is off by default.

XXX 2 items :

 * is -I a good choice ? maybe we need to prevent future --ignore-foo and
   add something like : --ignore=rsync_warning -I rsync_warning
 * the warning message is not enough explicit with the risk involved by
   --force usage

repmgr.h

@@ -55,6 +55,7 @@ typedef struct
 	char wal_keep_segments[MAXLEN];
 	bool verbose;
 	bool force;
+	bool ignore_rsync_warn;
 
 	char masterport[MAXLEN];