Add skip_if_not_implemented to the service client

This patch adds a decorator that wraps the service client methods adding
the ability to raise a skipException if a call to the API returns a
NotImplementedError or UnsupportedOptionError exception.

This is useful for running the tests against provider drivers that do
not support all of the Octavia API features.

Change-Id: I17d4be65130fadf97d6170d22fb07c72672b7573
diff --git a/requirements.txt b/requirements.txt
index b30b450..e5b93b8 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -16,4 +16,5 @@
 six>=1.10.0 # MIT
 tempest>=17.1.0 # Apache-2.0
 tenacity>=4.4.0 # Apache-2.0
+testtools>=2.2.0 # MIT
 keystoneauth1>=3.3.0 # Apache-2.0