Add python-netaddr to horizon requirements

Fixes bug 1036416

Change-Id: I97c1f193d30ad43e434ff33e4aacd1230be0341c
diff --git a/files/apts/horizon b/files/apts/horizon
index 53bddf0..2161ccd 100644
--- a/files/apts/horizon
+++ b/files/apts/horizon
@@ -21,3 +21,4 @@
 python-cherrypy3 # why?
 python-migrate
 nodejs
+python-netaddr