Allow agent to be installed from deb/rpm package

This will allow the agent to be installed from either rpm or dev packages.

By default this points to repo.zabbix.com and the appropriate directories.  Although, that can be overwritten with an attribute.

I had to create an attribute for the service name due to the packages having it as zabbix-agent and the templates using zabbix_agentd.  This should have no effect on any other install method as it maintains the same service name for any non packaged version.

Redhat/CentOS is untested as I don't have easy access to that OS currently.

I will be creating this install source for the server and web portions as well.
I will prefer something more generic like that:
```chef
service 'zabbix-agent' do
  service_name node['zabbix']['agent']['servicename']
  supports :status => true, :start => true, :stop => true, :restart => true
  action :nothing
end
```

It become easier to use `notifies` ;)