Add option to configure number of service ports in networks

Some services like portprober agent cannot be disabled via api,
so in tests like ip availability tests they should be explicitly
skipped from calculations.

Related-Prod: https://mirantis.jira.com/browse/PRODX-42203
Change-Id: I089bc013188079289914d98fd24cbfd3c4d91530
(cherry picked from commit ada996317cff04955f0c4bbc60c4e600689936f4)
diff --git a/tempest/config.py b/tempest/config.py
index eeedc9a..9e29ff1 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -887,6 +887,9 @@
                default=120,
                help="Total time in seconds to keep retrying a request that "
                     "returns HTTP 409 (Conflict)."),
+    cfg.IntOpt('service_ports_number',
+               default=0,
+               help="Number of neutron service ports created per network"),
 ]
 
 network_feature_group = cfg.OptGroup(name='network-feature-enabled',