Add iptables-based active/standby scenario test

This patch adds a tempest scenario test for active/standby topology.

This scenario takes a similar approach to scenario proposed in Change-Id
Ibcd5552a67cea650edc72bfaa986357267ca2407 with the difference that it
does not rely on amphora stats API. Instead, it uses iptables to log VIP
traffic and make assertions based on logging.

Co-Authored-By: Michael Johnson <johnsomor@gmail.com>

Change-Id: I24a6fc3387166ec6cdbd57a5ca9f63743748ec68
diff --git a/octavia_tempest_plugin/common/constants.py b/octavia_tempest_plugin/common/constants.py
index 0122424..54b23d1 100644
--- a/octavia_tempest_plugin/common/constants.py
+++ b/octavia_tempest_plugin/common/constants.py
@@ -84,6 +84,7 @@
 
 # Other constants
 ACTIVE = 'ACTIVE'
+PENDING_UPDATE = 'PENDING_UPDATE'
 ADMIN_STATE_UP_TRUE = 'true'
 ASC = 'asc'
 DELETED = 'DELETED'