Add mock module and additional documentation

This patch updates conf.py's mock_modules to include the required
python-novaclient module to ensure we have green tests.  This patch
also updates the openstack_vms.py description to be more explicit
about it's requirements.

docs/conf.py

@@ -30,7 +30,8 @@ MOCK_MODULES = [
     "i3pystatus.pulseaudio.pulse",
     "notmuch",
     "requests",
-    "bs4"
+    "bs4",
+    "python-novaclient"
 ]
 
 for mod_name in MOCK_MODULES:

i3pystatus/openstack_vms.py

@@ -9,6 +9,7 @@ class Openstack_vms(IntervalModule):
     """
     Displays the number of VMs in an openstack cluster in ACTIVE and
     non-ACTIVE states.
+    Requires: python-novaclient
     """
 
     settings = (