Allow overriding of class parameters in the hostgroup UI
When managing parameterized classes, parameters can be overridden by-host, directly from the host's page. The same would be very useful to have on the hostgroup page: the ability to add per-hostgroup overrides/defaults for class parameters.
fixes #2132 Smart Variables in hostgroup. 1. Show smart variables in hostgroup view. 2. Added the GUI to override smart variables values in host group.