Adops recipe to avoid outdated tox dependecies

Applies official workaround indicated by tox core maintainers for
avoiding tox-dev/tox#149 which allowed
tox to run with without updating dependencies declared in
external files.

Depends-On: https://review.openstack.org/#/c/617145/
Change-Id: I358b80d183a2667a04cfaac2054e101a9d7454ed

tox.ini

@@ -1,9 +1,12 @@
 [tox]
-minversion = 2.2
+minversion = 3.2.1
 envlist = linters, docs, py34, py35, py36, py27, cover
 skip_missing_interpreters = true
-# custom vars (no meaning to tox)
-install_test_deps = pip install -q -r test-requirements.txt
+
+# plugin to workaround https://github.com/tox-dev/tox/issues/149
+# where tox fails to update already existing virtualenvs
+tox_pip_extensions_ext_venv_update = true
+requires = tox-pip-extensions
 
 [testenv]
 setenv =
@@ -12,12 +15,13 @@ setenv =
  SUBUNIT_FORMATTER=tee testr_subunit_log
  VIRTUAL_ENV={envdir}
 usedevelop = True
-install_command = pip install {opts} {packages}
 # cleanup is needed mostly for dev environmnts and assures that if dev
 # is switching branch, no lefovers will impact execution.
 # No git commands here because test should run even from a source archive.
+deps =
+ -rrequirements.txt
+ -rtest-requirements.txt
 commands =
- {[tox]install_test_deps}
  - find . -type f -name "*.pyc" -delete
  - find . -type d -name "__pycache__" -delete
  # test that we can call jjb using both variants with same results
@@ -29,7 +33,6 @@ whitelist_externals =
 
 [testenv:tips]
 # tests what happens with unreleased version of dependencies, like python-jenkins
-install_command = pip install -U {opts} {packages}
 commands =
  bash -c "if [ -d {toxinidir}/../python-jenkins ]; then \
  pip install -q -U -e 'git+file://{toxinidir}/../python-jenkins#egg=python-jenkins' ; else \
@@ -41,7 +44,6 @@ setenv =
  {[testenv]setenv}
  PYTHON=coverage run --source jenkins_jobs --parallel-mode
 commands =
- {[tox]install_test_deps}
  stestr run {posargs}
  coverage combine
  coverage html -d cover
@@ -50,7 +52,6 @@ commands =
 [testenv:linters]
 basepython = python3
 commands =
- {[tox]install_test_deps}
  python -m pre_commit run --all
 
 # points to linters, kept only for developer convenience
@@ -73,7 +74,6 @@ commands = jenkins-jobs test -o .test/new/out/ .test/new/config/
 
 [testenv:docs]
 commands =
- {[tox]install_test_deps}
  python setup.py build_sphinx {posargs}
 
 [testenv:docs-linkcheck]