Work-around nose issue when skipping tests

As described in the comments, older versions of python (such as 2.6 on
RHEL) are not correctly skipping tests due to a mis-match of the
SkipTest exception between testtools and nose.  This is a work-around
while [1] is resolved.

This change pulls in a later version of testtools for the fix for [2].
It also creates a dependency on unittest2 which, as described in the
comments, is the fall-back SkipTest class to be used on python < 2.7

See [3]; Giulio Fidente did the work to diagnose this

[1] https://github.com/nose-devs/nose/pull/699
[2] https://bugs.launchpad.net/testtools/+bug/1190951
[3] https://review.openstack.org/#/c/33056
Skip test_volume_create_get_delete_as_clone

See upstream bug for root cause.  All these volume tests a susceptible
to this failure; more-so running in VM's like the CI jobs do

Change-Id: I2b2c2f7e83e1e1263bdd5b285a6d66fe0d09a36a