fix(tox): Add whitelist externals to tox.ini to avoid warnings

This PS adds whitelist externals to Armada's tox.ini to avoid
warnings that are issued while running tox -e py35 or tox -e pep8.
Example:

WARNING:test command found but not installed in testenv
  cmd: /usr/bin/find
  env: /opt/stack/armada/.tox/py35

This PS also resolves a critical error while running
tox -e coverage, because tox defaults to python2.7 for
the coverage job, and Armada's hapi module is not compatible
with python2.7. The fix involves setting the testenv's
basepython to python3.5.

Change-Id: Ice0074c3453c80290886f27a4518fbed01722572

tox.ini

@@ -9,8 +9,11 @@ deps=
 passenv=HTTP_PROXY HTTPS_PROXY http_proxy https_proxy NO_PROXY no_proxy
 setenv=
     VIRTUAL_ENV={envdir}
+basepython = python3.5
 usedevelop = True
 install_command = pip install {opts} {packages}
+whitelist_externals = find
+                      flake8
 commands =
   find . -type f -name "*.pyc" -delete
   python -V
@@ -38,7 +41,7 @@ commands =
 
 [testenv:coverage]
 commands =
-  pytest \
+  python -m pytest \
     --cov-branch \
     --cov-report term-missing:skip-covered \
     --cov-config .coveragerc \