core: add helper method for template version

some places in addVmTemplate command are checking for
baseTemplateId==null to understand if we add base template or template
version.

this patch encapsulate this logic into a method with a clear name.

Change-Id: I25229303fcee443191be1f2a1de84be345f96aff
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>