webadmin: Cluster sub general tab broken

- For certain locales the cluster general sub tab would not
  popuplate its values. This was due to an exception being
  thrown in the value parser of read only fields. Since these
  field are readonly they don't need a parser.
- Added a new EntityModelLabelEditor constructor without a parser
  that is meant for readonly fields.

Change-Id: I9c7a911d7a6ecfe79ee006be46e0ef8e627b97e3
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1157380
Signed-off-by: Alexander Wels <awels@redhat.com>