Medium: mysql: if appropriate, delete slave config on start

A node might be returning to the cluster after a crash and be configured
as a replication slave. When it returns and there is no master, we should
clear its slave configuration (i.e. change its master to none).

--HG--
extra : rebase_source : 95b1aef8642699f000bd7e4f1867c3d8b013a8f8