Use master branch of python-subunit to generate reports

Last released python-subunit (version 1.3.0) has
can't generate xml report file when running on Python 3

We need XML report for downstream CI as Jenkins
parse them for viewing detailed test case results.

As we are dropping down Python 2 support we must
use master branch version of python-subunit package
to generate reports until bug fix is not released
under via Pip.

Required python-subunit change:
 - https://github.com/testing-cabal/subunit/commit/58c3d364266e7550d3e00e34191eed7df2d36d13

Change-Id: I393855fbc81aad4d917e527a935bdb8ce6df13cb

report-requirements.txt

@@ -1,4 +1,4 @@
 # Requirements file for 'report' Tox environment
 
-os-testr>=1.0   # Apache 2.0
+os-testr>=1.0   # Apache-2.0
 stestr>=2.0     # Apache-2.0

tox.ini

@@ -198,8 +198,11 @@ commands =
 # --- CI report environments --------------------------------------------------
 
 [testenv:report]
-basepython=python2.7
+skipsdist=True
+usedevelop = False
 deps = -r {toxinidir}/report-requirements.txt
 passenv = {[base]passenv}
 setenv = {[base]setenv}
-commands = {toxinidir}/tools/ci/make_report
+commands =
+    pip install git+https://github.com/testing-cabal/subunit.git@master
+    {toxinidir}/tools/ci/make_report