core: Fix sql syntax for quota

calculateAllVdsGroupUsage function was fixed to be compatible with lower
postgres versions (verified on version 8.4.13)

Change-Id: I9eed6757ac4a9ed891792929be902aa628d62f59
Bug-Url: https://bugzilla.redhat.com/905891
Signed-off-by: Ofri Masad <omasad@redhat.com>