Use admin only common test case in admin action unit test cases

This patch use nova.tests.unit.api.openstack.compute.plugins.v3
.admin_only_action_common as the base class of AdminActionsTest
which can be shared by v2 and v21

Partially implements blueprint v2-on-v3-api
Change-Id: I16b59e8db59242b8e7b626cc77d0fff6bfc062e4
Merge "Use admin only common test case in admin action unit test cases"