Refactor options to enabled backends

Aodh can use only one database so having multiple options per backend
software is very redundant.

Also, gnocchi and prometheus are used for the same purpose so can use
a single option instead.

Change-Id: I134cf61d8137409ef5f5d959177e349781110eaa
diff --git a/.zuul.yaml b/.zuul.yaml
index de2a73b..cf4c861 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -54,8 +54,7 @@
             service_available:
               sg-core: True
             telemetry_services:
-              aodh_gnocchi: True
-              aodh_prometheus: True
+              metric_backends: gnocchi,prometheus
             telemetry:
               disable_ssl_certificate_validation: True
       tempest_test_regex: telemetry_tempest_plugin