Python 2.7 environment, mimicking the one used by RDO

Prompted by a recent issue:
https://review.rdoproject.org/r/#/c/32074/

New py27 environment approximates the steps taken during
the rdo packaging.

Signed-off-by: Jiri Podivin <jpodivin@redhat.com>
Change-Id: I9a74516f2b55f0ddf1f04637f0301c281903bb88

tox.ini

@@ -39,6 +39,29 @@ deps =
     -r {toxinidir}/doc/requirements.txt
 whitelist_externals = bash
 
+[testenv:py27]
+envtmpdir={envdir}/tmp
+skip_install = true
+commands_pre =
+  cp -r {toxinidir}/validations_common {envtmpdir}/validations_common
+  touch {envtmpdir}/validations_common/library/__init__.py
+  find {envtmpdir}/validations_common/tests/ -type f -exec sed -i -e 's/from unittest import mock/import mock/g' \{\} ;
+  find {envtmpdir}/validations_common/tests/ -type f -exec sed -i -e 's/from unittest.mock/from mock/g' \{\} ;
+  find {envtmpdir} -type f -name 'setup.cfg' -exec sed -i -e 's/python-requires = >=3.6/python-requires = >=2.7/g' \{\} ;
+commands =
+  python2.7 -m unittest discover -s {envtmpdir}/validations_common/tests
+deps =
+    -c {env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/train/upper-constraints.txt}
+    -r {toxinidir}/requirements.txt
+    -r {toxinidir}/test-requirements.txt
+    ansible>=2
+whitelist_externals =
+  bash
+  cp
+  find
+  sed
+  touch
+
 [testenv:bindep]
 # Do not install any requirements. We want this to be fast and work even if
 # system dependencies are missing, since it's used to tell you what system