Require title of mysql_grant resource to match form user/table

This addresses https://tickets.puppetlabs.com/browse/MODULES-1040.
The user parameter is required to have the form username@host. A grant
is identified in the instances method by a name of the form
username@host/table. The resource will fail to be identified as already
existing if the name given to the resource does not match this form.

I'm happy to dig into this more if there are other suggestions for how to fix this.