Support bootstraping a Galera cluster with mysqld_multi

Starting a Galera cluster with mysqld_multi currently requires to manually add (and later remove) --wsrep-new-cluster in the config block of the mysqld instance that is supposed to become the first cluster node.

I suggest to add a --new-cluster option to mysqld_multi that will add --wsrep-new-cluster to the mysqld command line of each server process started by mysqld_multi --new-cluster start ...