Replace reference to deprecated [service_available] aodh_plugin

This option was deprecated in favor of the new aodh option. However
the old name is still used by code which causes the following warning
message.

WARNING oslo_config.cfg [-] Config option service_available.aodh_plugin
is deprecated. Use option service_available.aodh instead.

Change-Id: If10cec87a2f07f9dbe09005bb22070ca7a6e9364
diff --git a/telemetry_tempest_plugin/aodh/api/base.py b/telemetry_tempest_plugin/aodh/api/base.py
index 29b1d61..816f204 100644
--- a/telemetry_tempest_plugin/aodh/api/base.py
+++ b/telemetry_tempest_plugin/aodh/api/base.py
@@ -29,7 +29,7 @@
     @classmethod
     def skip_checks(cls):
         super(BaseAlarmingTest, cls).skip_checks()
-        if not CONF.service_available.aodh_plugin:
+        if not CONF.service_available.aodh:
             raise cls.skipException("Aodh support is required")
 
     @classmethod