Implement simple failover script using GTID

GTID allows to automatically fail-over a master by promoting a slave as the
new master, bringing it up-to-date with any other slaves and switching
existing slaves to use the new master. The procedure is described in the
documentation.

https://mariadb.com/kb/en/global-transaction-id/#start-slave-until-master_gtid_posxxx

We need a script that implements this procedure. The user can then run this
script to promote a slave as a new master, simply by supplying connection
information for all involved servers.