add python-mock to testonly package list

Python-mock is needed by most of OpenStack for testability. Add
this is a testonly package. This should fix current issues with
functional tests in OpenStack that use devstack, but not tox to
run their tests.

Closes-Bug: #1341507

Change-Id: I26cbe42ace75554bae993a6d849c3b4ac649c78e
diff --git a/files/apts/general b/files/apts/general
index 90529e5..f3cab59 100644
--- a/files/apts/general
+++ b/files/apts/general
@@ -20,6 +20,7 @@
 tar
 python-cmd2 # dist:precise
 python-dev
+python-mock # testonly
 python2.7
 bc
 libyaml-dev