Fix inheriting tags from resource pool

https://bugzilla.redhat.com/show_bug.cgi?id=1164386

"Resource Pool" needs to point to a valid method in `VmOrTemplate`. `VmOrTemplate#parent_resource_pool` is the existing method while `VmOrTemplate#resource_pool` does not exist. 

Another way to fix the problem is to add an alias `resource_pool` for the existing `parent_resource_pool` method in `VmOrTemplate`. This way any user already defined policies will start to work. The down side of the way is exposing a new (alias) method only for this bug.