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.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1075074
Change-Id: I25229303fcee443191be1f2a1de84be345f96aff
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>