Bug #6483: http://projects.theforeman.org/issues/6483

Bug 6483 is more visible when a RHEV/oVirt compute resource have more than 1 cluster. 

In RHEV, NICs are attached to cluster. The Network dropdown is populated when a cluster is selected. Problem is when the ovirt forms (_ovirt.html.erb, ovirt/_network.html.erb) are loaded initially, "selected_cluster" is always nil (when 2 or more cluster is available in the compute resource) unless someone manually pick another cluster in the Cluster dropdown, which sets the selected_cluster via AJAX call. 

The fix is to pass the save selected_cluster directly to the views.  
nitpick, could you move the selected cluster to `args_for_compute_resource_partial`?
there are two more args there that both check if host is nil and it could be checked once for all three args