Add py27 testenv for tox.ini

Since upstream requirements is removing support of py27 but
TripleO jobs still runs on py27 and which is going to break
the deps installation.

In order to fix, we can create a seperate testenv py27
to handle that and it should not depends on upstream
constriants.

Related-Bug: #1861803
Change-Id: I61365503cc1c70644d6520f54646492359fa4e44

tox.ini

@@ -11,6 +11,13 @@ deps =
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
+[testenv:py27]
+basepython = python2.7
+deps =
+       -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+commands = python setup.py testr --slowest --testr-args='{posargs}'
+
 [testenv:venv]
 basepython = python3
 commands = {posargs}