Resurrect global .distinct() call (mostly)

This mostly reverts 3c67971e78a12bd94536aa5464f0bc1ea46ba1ee with
the minor difference that we only apply this when we're filtering,
which is apparently necessary without some notable overhaul since
the filtering we're doing will get stuck in as filters, which will
generate inner joins, which can result in duplicates if the thing
we're joining with is a one to many or many to many, which most
things are.

With this patch we still need to be generating naturally distinct
querysets with any `get_queryset` methods, which will still be much
more effecient when filtering is not involved.

This fixes #2032 and probably a bunch of other undiscovered issues.

awx/api/filters.py

@@ -219,7 +219,7 @@ class FieldLookupBackend(BaseFilterBackend):
                     else:
                         q = Q(**{k:v})
                     queryset = queryset.filter(q)
-                queryset = queryset.filter(*args)
+                queryset = queryset.filter(*args).distinct()
             return queryset
         except (FieldError, FieldDoesNotExist, ValueError), e:
             raise ParseError(e.args[0])