Stop using deprecated tempest options

With the merge of 'Deprecate credential config options' in tempest,
(Change-Id: Ia5132c5cb32355d6f26b8acdd92a0e55a2c19f41)
admin credentials options have been moved from [identity] group to
[auth] group in tempest/config.py. This makes no difference for cinder
tempest tests.  However manila has separate tempest tests and
in manila_tempest_tests/tests/api/base.py, BaseSharesAdminTest always
gets admin credentials from identity group. It will fail with the
changed configuration options.

Closes-Bug: #1504052
Change-Id: If329a6563df2028a18c3fedea53065688e75339e
diff --git a/manila_tempest_tests/tests/api/base.py b/manila_tempest_tests/tests/api/base.py
index a8f2b83..7beb289 100644
--- a/manila_tempest_tests/tests/api/base.py
+++ b/manila_tempest_tests/tests/api/base.py
@@ -713,9 +713,14 @@
 
     @classmethod
     def resource_setup(cls):
-        cls.username = CONF.identity.admin_username
-        cls.password = CONF.identity.admin_password
-        cls.tenant_name = CONF.identity.admin_tenant_name
+        if hasattr(CONF.identity, 'admin_username'):
+            cls.username = CONF.identity.admin_username
+            cls.password = CONF.identity.admin_password
+            cls.tenant_name = CONF.identity.admin_tenant_name
+        else:
+            cls.username = CONF.auth.admin_username
+            cls.password = CONF.auth.admin_password
+            cls.tenant_name = CONF.auth.admin_tenant_name
         cls.verify_nonempty(cls.username, cls.password, cls.tenant_name)
         cls.os = clients.AdminManager()
         admin_share_network_id = CONF.share.admin_share_network_id