Use tempest.lib code in tempest

Now that tempest_lib code is copied back into tempest, stop using
tempest_lib in tempest, and start using the copied code.

Remove the dependency to tempest_lib from requirements, and drop
the script to use tempest_lib in tempest. Add os-testr to the
test-requirements.

Partially implements bp tempest-lib-reintegration

Change-Id: I21ab5fe6349f72c98ac9f960a29bf62e813f8b1b
diff --git a/tempest/common/cred_client.py b/tempest/common/cred_client.py
index 494bcee..edc51ca 100644
--- a/tempest/common/cred_client.py
+++ b/tempest/common/cred_client.py
@@ -14,9 +14,9 @@
 
 from oslo_log import log as logging
 import six
-from tempest_lib import auth
-from tempest_lib import exceptions as lib_exc
 
+from tempest.lib import auth
+from tempest.lib import exceptions as lib_exc
 from tempest.services.identity.v2.json import identity_client as v2_identity
 
 LOG = logging.getLogger(__name__)