Adjust scenario tests for NotImplemented skip

This is a patch to restructrue the scenario tests to use the
new skip_if_not_implemented capability.

Change-Id: I49a7fb6650030f2a1115c6d42442062bd33415fd
diff --git a/octavia_tempest_plugin/common/constants.py b/octavia_tempest_plugin/common/constants.py
index 16d6cdf..ee4c16a 100644
--- a/octavia_tempest_plugin/common/constants.py
+++ b/octavia_tempest_plugin/common/constants.py
@@ -124,6 +124,7 @@
 # Protocols
 HTTP = 'HTTP'
 HTTPS = 'HTTPS'
+PROXY = 'PROXY'
 TCP = 'TCP'
 TERMINATED_HTTPS = 'TERMINATED_HTTPS'
 UDP = 'UDP'