Separate token client from identity client

Identity 'TokenClientJSON' is being derived from 'IdentityClientJSON'
which is not needed. 'TokenClientJSON' class can be directly derived from
ServiceClient.

This patch makes the above inheritance changes and separate token
clients (V2 & v3) in separate file.

This will helpful in moving services client to tempest-lib.

Change-Id: I25186dc1fde57cd7fd16784aa6baf2f8d2f0a46d
diff --git a/tempest/tests/test_credentials.py b/tempest/tests/test_credentials.py
index 1abb941..6e447d6 100644
--- a/tempest/tests/test_credentials.py
+++ b/tempest/tests/test_credentials.py
@@ -21,8 +21,8 @@
 from tempest.common import tempest_fixtures as fixtures
 from tempest import config
 from tempest import exceptions
-from tempest.services.identity.json import identity_client as v2_client
-from tempest.services.identity.v3.json import identity_client as v3_client
+from tempest.services.identity.json import token_client as v2_client
+from tempest.services.identity.v3.json import token_client as v3_client
 from tempest.tests import base
 from tempest.tests import fake_config
 from tempest.tests import fake_identity