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
diff --git a/tempest/api/identity/test_extension.py b/tempest/api/identity/v2/test_extension.py
similarity index 95%
rename from tempest/api/identity/test_extension.py
rename to tempest/api/identity/v2/test_extension.py
index 7095181..c538c14 100644
--- a/tempest/api/identity/test_extension.py
+++ b/tempest/api/identity/v2/test_extension.py
@@ -17,7 +17,7 @@
 from tempest.lib import decorators
 
 
-class ExtensionTestJSON(base.BaseIdentityV2AdminTest):
+class ExtensionTestJSON(base.BaseIdentityV2Test):
 
     @decorators.idempotent_id('85f3f661-f54c-4d48-b563-72ae952b9383')
     def test_list_extensions(self):