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.
Merge pull request #522 from cmurphy/fix_mysql_grant_MODULES-1040

Require title of mysql_grant resource to match form user/table