Added zabbix::_repository for Zabbix official packages

Supports both apt and yum. Set node['zabbix']['major_version'] for the major
version of Zabbix, default is 2.0.
Added zabbix::_repository for Zabbix official packages

Supports both apt and yum. Set node['zabbix']['major_version'] for the major
version of Zabbix, default is 2.0.
Rename service[zabbix_agentd] to service[zabbix-agentd]

Rename service to make the zabbix::agent_package recipe cleaner, but still keep
backwards compatibility with the source install.
Add package install support.

Install zabbix-agent package from the official Zabbix repository.
Set include dir to the default from the package.