Clarify dependency for database handling

[stefanandres]

See #505

[bastelfreak]

Thanks for the bugfix @stefanandres !

[bastelfreak]

@stefanandres can you take a look at the travis failures?

[stefanandres]

Yes, of course :-)

[stefanandres]

Ah, in fact I was using it wrong.

The workflow should be:
Use the following classes (in this Order):

• zabbix::database
• mysql::server
• zabbix::server

Before I was using zabbix::database::mysql directly, but server.pp already requires on the package. To what is to be done is to make zabbix::database::mysql a private class. I'll adjust the PR for this.
This will help new users not to confuse it also.

[bastelfreak]

thanks!