Fix MySQL 5.6 compatibility

In MySQL 5.6, the root account created by default during MySQL
installation has the PROXY ... WITH GRANT OPTION privilege for ''@'',
that is, for all users.

The mysql_user module tries to revoke this privilege, but this fails:
_mysql_exceptions.ProgrammingError: (1064, "You have an error in your
SQL syntax; check the manual that corresponds to your MySQL server
version for the right syntax to use near '''@'' FROM 'root'@'localhost''
at line 1")

Quick fix: don't revoke privilege if user is root and the privilege to
revoke contains PROXY.
Merge pull request #3177 from stintel/mysql56-compat

Fix MySQL 5.6 compatibility