core: Fix quota release for Remove-Template

Quota resources are not released when deleting a template. This happened
because the template disk list was not initialized before the quota
consumption parameters check.

Added initialization of the list when before quota consumption
parameters check.

Change-Id: Iaf9326bfb81df2cdf1132f693d30014056df5d66
Bug-Url: https://bugzilla.redhat.com/913452
Signed-off-by: Ofri Masad <omasad@redhat.com>