Mysql hook master-relation-changed fails when removing a slave relation

[Darryl Weaver]

When removing a Mysql slave database relation with the master database the master databases relation-departed hook runs but fails with:
2014-08-19 16:59:56 INFO master-relation-departed + mysql -uroot -pPASSWORD -e REVOKE REPLICATION SLAVE, REPLICATION CLIENT ON *.* FROM `slave-db`@``
2014-08-19 16:59:56 INFO master-relation-departed ERROR 1141 (42000) at line 1: There is no such grant defined for user 'slave-db' on host ''
2014-08-19 16:59:56 ERROR juju.worker.uniter uniter.go:486 hook failed: exit status 1

This occurs because the $remote_ip variable is empty and so there is no matching user or grant on the mysql master.