Supports DEFAULT group opts in TempestPrivateConfig

Options from the DEFAULT group are not accessible via the config
proxy. Adding support for DEFAULT group opts in TempestPrivateConfig to
solve that.

Partially implements bp:test-accounts

Change-Id: Ia80c703080870427bbacc12a239e385d9927631f
diff --git a/tempest/config.py b/tempest/config.py
index af6dd7a..266c619 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1079,6 +1079,10 @@
 
     DEFAULT_CONFIG_FILE = "tempest.conf"
 
+    def __getattr__(self, attr):
+        # Handles config options from the default group
+        return getattr(cfg.CONF, attr)
+
     def _set_attrs(self):
         self.auth = cfg.CONF.auth
         self.compute = cfg.CONF.compute