Move test_extension and use base for non-admin

"list extensions" API is implemented on V2 API only in Keystone,
and the corresponding test doesn't require admin credential.
So this patch moves the test to v2 directory and makes the test
use the base test for non-admin.

Change-Id: I1ebb82e4e0593ab904bd77e231e5f7b8f275c872
1 file changed