LWRP for database installation

Currently database credentials are stored on the chef server via node.set

This is a first pass in an effort to make that not the case.

In Progress: 
- [x] Resource for zabbix_database
- [x] Default Provider that handles MySql and RDS because that's all the cookbook currently does
- [x] Allow definition of other providers for database
- [x] Don't download the zabbix package more than once per node
- [x] Make it very clear in the README that zabbix::database recipe doesn't treat credentials securely