Fix image namespace test logging

This commit fixes the BaseV2ImageTest create_namespace instance
function to use self instead of cls, so that find_test_caller
can look up the object's fields while inspecting the call frame.

Change-Id: Iabfdcb90a8d37c7bcdefdb2f99d60bf034e24fd9
Closes-Bug: 1748064
diff --git a/tempest/api/image/base.py b/tempest/api/image/base.py
index 142e3c2..4911ba4 100644
--- a/tempest/api/image/base.py
+++ b/tempest/api/image/base.py
@@ -142,16 +142,17 @@
         cls.schemas_client = cls.os_primary.schemas_client
         cls.versions_client = cls.os_primary.image_versions_client
 
-    def create_namespace(cls, namespace_name=None, visibility='public',
+    def create_namespace(self, namespace_name=None, visibility='public',
                          description='Tempest', protected=False,
                          **kwargs):
         if not namespace_name:
             namespace_name = data_utils.rand_name('test-ns')
         kwargs.setdefault('display_name', namespace_name)
-        namespace = cls.namespaces_client.create_namespace(
+        namespace = self.namespaces_client.create_namespace(
             namespace=namespace_name, visibility=visibility,
             description=description, protected=protected, **kwargs)
-        cls.addCleanup(cls.namespaces_client.delete_namespace, namespace_name)
+        self.addCleanup(self.namespaces_client.delete_namespace,
+                        namespace_name)
         return namespace