Avoid weird test error when mox is missing

Fail early if you can't import mox, rather than return
a cryptic and undecipherable error. Fixes bug 810424.

Change-Id: Ie56d3e7447bbba841fdc882cb1f8f70e234812ef

nova/tests/__init__.py

@@ -37,6 +37,7 @@ setattr(__builtin__, '_', lambda x: x)
 
 
 def setup():
+    import mox  # Fail fast if you don't have mox. Workaround for bug 810424
     import os
     import shutil