core: Use validation only if Template is not blank

The validation of, same DC when creating a VM from a Template,
should only be validated when creating a VMs which does not have
Blank Template as their based on image.

Blank Template is the default Template for every new created VM, and it is
hosted on the default Data Center

Change-Id: I06a0c911c50f6ff25d9dca358c6a199fa0cdbb59
Bug Url: https://bugzilla.redhat.com/1125196
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>