repmgr: don't error out on superfluous command line options

When parsing command line arguments in check_parameters_for_action(),
create warnings for paramters supplied but not required (e.g. -D/--data-dir
for MASTER REGISTER), rather than fail with error(s), as the
presence of the parameters won't cause any problems.

Errors will still be raised for required-but-missing parameters, of course.

TODO

@@ -60,9 +60,5 @@ Usability improvements
   while running a backup, an attempt can be made to execute pg_stop_backup()
   on the primary, to prevent an orphaned backup state existing.
 
-* repmgr: when parsing command line arguments in check_parameters_for_action(),
-  don't error out on parameters which aren't relevant for the particular
-  action; instead collate into a list of warnings
-
 * repmgr: when unregistering a node, delete any entries in the repl_monitoring
   table.