Remove policy check in db layer for aggregates

Since there already are policy checks in v2 and v3 api layer,
no need to check again at db layer. This patch remove
that check for aggregates. Also added policy test
on api layer for v2 API(v3 already have),
removed 2 test cases in db layer focused admin context.

Change-Id: Ied1fd109be104a9539d1cad7937455852e20ffb2
Implements: blueprint v3-api-policy
Merge "Remove policy check in db layer for aggregates"