Health check should verify replication

[dfredell]

As stated in

mysql/bin/manage.py

Line 156 in a0ea5d6

# TODO: we should make this check actual replication health

lets actually check the health of the replication.

I have ended up in this state https://blog.tinned-software.net/fix-mysql-replication-error/ where SHOW SLAVE STATUS \G; returns
Last_Error: Error 'Can't create database 'my_db'; database exists' on query. Default database: 'my_db'. Query: 'CREATE SCHEMA my_db'

As Gerhard from the link above recommends, we should be checking Slave_IO_Running == Yes && Slave_SQL_Running == Yes && Seconds_Behind_Master != NULL