Replace nosetests with testr in tox.ini.

Config tox.ini updated to run testr instead of nosetests.
File test-requirements.txt modified respectively.
Config .testr.conf added.

Testr allows for more flexible testing, more consistent use of the
unittest protocol and, most importantly, faster testing via parallelism.

Part of blueprint run-tests-script.

Change-Id: Iba847d430a18458a1a68e6ef07541d29c0c01c64
Merge "Replace nosetests with testr in tox.ini."