Prepare for enabling H302 (identity,volume,etc.)

We can use H302 rule but ignore it now. This commit prepares for
enabling H302 rule. But this commit modifies tempest/api/{identity,
orchestration, volume, utils} only and excludes some violations[1]
because if we fix all of them at one time, it's hard to merge this
commit.

[1]
 tempest.test.attr
 tempest.common.rest_client.RestClient
 tempest.common.rest_client.RestClientXML
 tempest.services.compute.xml.common.xml_to_json
 tempest.services.compute.xml.common.Element
 tempest.services.compute.xml.common.Document
 tempest.services.compute.xml.common.Text
 tempest.services.compute.xml.common.XMLNS_11
 tempest.services.compute.xml.common.XMLNS_V3
 tempest/clients.py

Change-Id: I811144df35a5a4d9a256cfaba1dcd28bbfbccb27
diff --git a/tempest/api/utils.py b/tempest/api/utils.py
index c62dc8d..00c93b7 100644
--- a/tempest/api/utils.py
+++ b/tempest/api/utils.py
@@ -15,7 +15,7 @@
 
 """Common utilities used in testing."""
 
-from tempest.test import BaseTestCase
+from tempest import test
 
 
 class skip_unless_attr(object):
@@ -30,7 +30,7 @@
             """Wrapped skipper function."""
             testobj = args[0]
             if not getattr(testobj, self.attr, False):
-                raise BaseTestCase.skipException(self.message)
+                raise test.BaseTestCase.skipException(self.message)
             func(*args, **kw)
         _skipper.__name__ = func.__name__
         _skipper.__doc__ = func.__doc__