webadmin: Hide custom policy properties section if empty

This patch updates the Cluster policy popup dialog
used to configure ClusterPolicy.

The section for custom parameters stays hidden if there
are no parameters set and none are announced by the
currently used PolicyUnits.

Change-Id: I41cc951ad977f24821d81e909ba958064695903e
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1064397
Signed-off-by: Martin Sivak <msivak@redhat.com>