Allow tests to run outside tox

When using a test runner outside of tox (for example
using py.test directly against test files, one of several
ways to enable faster and more specific testing)
oslo.messaging.conffixture is not imported and text/base fails
to compile.

This change gets things working and otherwise has no impact.

Change-Id: Ibac6ae0a73aa55522b248a26b9a4b82455f8fce6

ceilometer/tests/base.py

@@ -21,7 +21,7 @@
 import os.path
 
 import eventlet
-import oslo.messaging
+import oslo.messaging.conffixture
 import six
 from testtools import testcase