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.

database/mysql_user

@@ -153,6 +153,7 @@ def user_mod(cursor, user, host, password, new_priv):
         # the new specification, then revoke all privileges on it.
         for db_table, priv in curr_priv.iteritems():
             if db_table not in new_priv:
+                if user != "root" and "PROXY" not in priv:
                     privileges_revoke(cursor, user,host,db_table)
                     changed = True