[Tests] Fix for flake8 in tox

Force install flake8 in tox to avoid the system flake8 being used if
available.

Remove unneeded whitelist entries

tox.ini

@@ -90,22 +90,19 @@ commands =
     python -c "import subprocess, sys; output = subprocess.check_output('isort -q --diff --recursive deluge docs packaging *.py', shell=True); print output; sys.exit(len(output) != 0)"
 
 [testenv:flake8]
+# Force flake8 pip install as system flake8 uses hardcoded python path which imports the wrong libraries.
+install_command = pip -v install --ignore-installed {opts} {packages}
 setenv = {[testenv]setenv}
-whitelist_externals =
-    {[testenv]whitelist_externals}
-    flake8
 deps =
     {[testenv]deps}
     flake8
     pep8-naming
-commands = flake8
+commands =
+    flake8 --version
+    flake8
 
 [testenv:flake8-complexity]
 setenv = {[testenv]setenv}
-whitelist_externals =
-    {[testenv]whitelist_externals}
-    flake8
-    sh
 deps =
     {[testenv:flake8]deps}
     mccabe