Use unittest2.TestCase.assertDictEqual.
Use unittest2.TestCase.assertDictEqual.

The standard library version of unittest in CentOS 6 does not include an
assertDictEqual method. However, the third-party package unittest2 is a
backport of unittest features from later versions of Python.

This change modifies mlab_test to import unittest2 and use
assertDictEqual where appropriate.

Cleanup import order and third-party comment.

m-lab/collectd-mlab,stephen-soltesz/collectd-mlab,m-lab/collectd-mlab,stephen-soltesz/collectd-mlab