core: add distinct keyword to search

currently, the search can return duplicate results,
for example when template has more than one disk,
or disks are on different storage domains.

this patch adds a 'distinct' to the sql the search mechanism creates,
in order to remove any duplicates that return from the query.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1130915
Change-Id: I070900dc9e1064c98f8b342f26dbc3c23ba8c4a4
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>