Fix mysql::server::monitor mysql_grant privileges

This PR fixes invalid privileges being used with the mysql_grant type as well as makes the module conform to mysql::backup's style.  Should be released as a bug fix as the class' behavior does not change.

This bug is also present in the 2.0.x releases.

This is the type of error that would be generated before this fix.

```
Error: Execution of '/usr/bin/mysql --defaults-file=/root/.my.cnf -e GRANT PROCESS_PRIV, SUPER_PRIV ON *.* TO 'zabbix-agent'@'localhost'' returned 1: ERROR 1064 (42000) at line 1: 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 'PROCESS_PRIV, SUPER_PRIV ON *.* TO 'zabbix-agent'@'localhost'' at line 1
```