block testtools 1.2.0

We're now seeing the following failure in all tempest runs:

subunit.run discover: error: no such option: --list

This coincides with testtools 1.2.0 release, so block it to try to get
everything functioning again.

Conflicts:
	global-requirements.txt

Change-Id: I2f8737b44c703c3094d6bbb6580993f86a571934
(cherry picked from commit 81d479cd94da651ba2822c57e502544bdc86dfa0)
Merge "block testtools 1.2.0" into stable/icehouse