Convert image and identity api tests to use global CONF object

This commit takes all the uses of config in the image and identity api
tests and converts them to use the global CONF object.

Partially implements bp config-cleanup

Change-Id: Ic305ce82d2bd5d0ef8293fe25c6a4c835633b0e8
diff --git a/tempest/api/identity/admin/v3/test_trusts.py b/tempest/api/identity/admin/v3/test_trusts.py
index 0bded78..1bebad4 100644
--- a/tempest/api/identity/admin/v3/test_trusts.py
+++ b/tempest/api/identity/admin/v3/test_trusts.py
@@ -31,7 +31,7 @@
         if not CONF.identity_feature_enabled.trust:
             raise self.skipException("Trusts aren't enabled")
 
-        self.trustee_username = self.config.identity.alt_username
+        self.trustee_username = CONF.identity.alt_username
         self.trust_id = None
 
     def tearDown(self):