core: Not possible to edit an instance type

A regression introduced in 176bafa8f9ecdc7bf4f06cb0bebd7c2044de2832

The mOldTemplate has to be loaded also when the cluster is null (the case of
instance type).

Change-Id: I3229350827f2d4dc90ba815007d7cbf1e73b74bc
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>