MDEV#4138: Galera: mysqld_safe doesn't pass on wsrep_provider to mysqld

In mysqld_safe script, during the processing of command line
options, the supplied wsrep_provider option was not getting
appended to the set of mysqld options. Hence, wsrep plugin
would never receive the path for wsrep provider.

Fixed by adding logic to append the wsrep_provider option
to the set of mysqld options.

scripts/mysqld_safe.sh

@@ -328,6 +328,7 @@ parse_arguments() {
         then
           wsrep_restart=1
         fi
+        append_arg_to_args "$arg"
         ;;
       --help) usage ;;