Mysql_grant fix

Type mysql_grant was limited to work only with resource names foo@localhost/*.* or foo@localhost/bar.* but NOT with root@localhost/@ which is the parsed value of "GRANT PROXY ON ''@'' TO 'root'@'localhost' WITH GRANT OPTION" grant line.

New spec test was created for the type mysql_grant.rb

defaults_file methods removed from every provider (old & new) and moved
the method to the global mysql.rb which is now included in every provider
file.