Update "delete_by_query" method to return number of deleted objects.
Update "delete_by_query" method to return number of deleted objects.

We now depend and use newer version of mongoengine where "delete()" returns
number of deleted / affected objects (previously in older versions it returned
None).

StackStorm/st2,Plexxi/st2,lakshmi-kannan/st2,StackStorm/st2,StackStorm/st2,lakshmi-kannan/st2,tonybaloney/st2,tonybaloney/st2,Plexxi/st2,Plexxi/st2,lakshmi-kannan/st2,peak6/st2,nzlosh/st2,nzlosh/st2,StackStorm/st2,nzlosh/st2,peak6/st2,tonybaloney/st2,Plexxi/st2,nzlosh/st2,peak6/st2