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/telemetry_tempest_plugin/config.py b/telemetry_tempest_plugin/config.py
index ef43cf7..be3f6d4 100644
--- a/telemetry_tempest_plugin/config.py
+++ b/telemetry_tempest_plugin/config.py
@@ -30,6 +30,10 @@
                   cfg.BoolOpt('gnocchi',
                               default=True,
                               help="Whether or not Gnocchi is expected to be"
+                                   "available"),
+                  cfg.BoolOpt('sg_core',
+                              default=True,
+                              help="Whether or not sg-core is expected to be"
                                    "available")]
 
 telemetry_group = cfg.OptGroup(name='telemetry',