Add tests for allowed CIDRs in listeners

This patch adds API and scenario tests for testing allowed CIDRS in
listeners introduced in API version 2.12 (included in Train release).

Change-Id: Ibe677e046afc16f038ccacb10e5fe62802828581
diff --git a/octavia_tempest_plugin/common/constants.py b/octavia_tempest_plugin/common/constants.py
index 0e24283..16d6cdf 100644
--- a/octavia_tempest_plugin/common/constants.py
+++ b/octavia_tempest_plugin/common/constants.py
@@ -14,6 +14,7 @@
 
 # API field names
 ACTIVE_CONNECTIONS = 'active_connections'
+ALLOWED_CIDRS = 'allowed_cidrs'
 AVAILABILITY_ZONE = 'availability_zone'
 AVAILABILITY_ZONE_DATA = 'availability_zone_data'
 AVAILABILITY_ZONE_PROFILE_ID = 'availability_zone_profile_id'