Initial class creds creation in test base class

Each test class may now define at class level which credentials
are needed, and they will be allocated automatically by the base
class. To start using this a test class that requires network
resources must implement the setup_credentials method and
define the required resources before super is invoked.

In this patch this only affects the creation of credentials
as defined in the various base classes. Other tests will be
migrated as part of the resource-cleanup bp. Note that this
changes baremetal, identity and orchestration tests to
honour the tenant isolation settings.

Partially-implements: bp resource-cleanup

Change-Id: Id36a6ebddb618a78cee7025c9537cd1e2746190e
diff --git a/tempest/api/volume/test_volume_transfers.py b/tempest/api/volume/test_volume_transfers.py
index 4acab39..bcd3fa9 100644
--- a/tempest/api/volume/test_volume_transfers.py
+++ b/tempest/api/volume/test_volume_transfers.py
@@ -16,8 +16,6 @@
 from testtools import matchers
 
 from tempest.api.volume import base
-from tempest import clients
-from tempest.common import credentials
 from tempest import config
 from tempest import test
 
@@ -26,21 +24,7 @@
 
 class VolumesV2TransfersTest(base.BaseVolumeTest):
 
-    @classmethod
-    def skip_checks(cls):
-        super(VolumesV2TransfersTest, cls).skip_checks()
-        if not credentials.is_admin_available():
-            msg = "Missing Volume Admin API credentials in configuration."
-            raise cls.skipException(msg)
-
-    @classmethod
-    def setup_credentials(cls):
-        super(VolumesV2TransfersTest, cls).setup_credentials()
-        # Add another tenant to test volume-transfer
-        cls.os_alt = clients.Manager(cls.isolated_creds.get_alt_creds())
-        # Add admin tenant to cleanup resources
-        creds = cls.isolated_creds.get_admin_creds()
-        cls.os_adm = clients.Manager(credentials=creds)
+    credentials = ['primary', 'alt', 'admin']
 
     @classmethod
     def setup_clients(cls):