Separate prometheus related tests

We have an issue, that tempest doesn't distinguish between
branches. New features aren't available on old branches and
so we can't execute testing, which involves sg-core or prometheus
on old branches. Separating sg-core and prometheus related tests
allows us to use tempest regex to execute those tests only on
branches, which have the required features.

This also fills in service_available requirements, to make it
clear which tests are runable on prometheus based storage
and which are runable on gnocchi based storage.

Change-Id: Ic5bea08295d8c85303c676706415e093625ee16f
diff --git a/.zuul.yaml b/.zuul.yaml
index 211fc57..fdeb80f 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -93,6 +93,13 @@
         USE_PYTHON3: True
         TEMPEST_PLUGINS: '"/opt/stack/telemetry-tempest-plugin /opt/stack/heat-tempest-plugin"'
         GNOCCHI_ARCHIVE_POLICY_TEMPEST: "ceilometer-high-rate"
+        # NOTE(jwysogla): We can define both of the variables. In versions, where
+        # the ceilometer devstack plugin doesn't support the CEILOMETER_BACKENDS,
+        # it'll just ignore it and use the CEILOMETER_BACKEND. In versions, where
+        # CEILOMETER_BACKENDS is supported, the ceilometer devstack plugin will
+        # just try to merge the variables, so the final contents in this casse will
+        # be "gnocchi,sg-core"
+        CEILOMETER_BACKEND: "gnocchi"
         CEILOMETER_BACKENDS: "gnocchi,sg-core"
         CEILOMETER_PIPELINE_INTERVAL: 15
         CEILOMETER_ALARM_THRESHOLD: 6000000000