Marks mysql slave read-only

When a slave is attached to a master site, this change will
mark the slave site as read-only to prevent the slave site
from being updated by users.  Only users with root privileges
or slave priveleges will be able to update the database.

Change-Id: Idb6c54b85a23af9a468ff9d62b3110fa72ff7da0
Closes-bug: #1359392
Merge "Marks mysql slave read-only"