Add dynagroup expression template for RHQ Agents

[Thomas Segismont]

Hi everyone,

I just pushed to master a change to add a dynagroup expression template 
for RHQ Agents [1].

This is to help us to quickly build a dynagroup of all RHQ agents, which 
can be useful when you want to run operations like "Update all plugins" 
on all resources of this type.

Regards,
Thomas

[1] 
https://github.com/rhq-project/rhq/commit/db9ec0e0abdc10589d14d41c770b35b...