tox: Rename flake8 -> linters

This is expected by the gate. There's probably some configuration
somewhere that can be tweaked but this is easier. We need to pin our
hacking version because tempest uses 'pep8' in its custom hacking
checks and this is not installed with the latest version of hacking [1].

[1] https://github.com/openstack-dev/hacking/commit/24d7eb6a40e5

Change-Id: I1b5b34b4d277d1e412120736d05c2816f9a572dd

test-requirements.txt

@@ -1,5 +1,5 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-hacking
+hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 oslotest

tox.ini

@@ -14,9 +14,9 @@ deps =
   -r{toxinidir}/test-requirements.txt
 commands =
   find . -type f -name "*.pyc" -delete
-  stestr run '{posargs}'
+  stestr run {posargs}
 
-[testenv:flake8]
+[testenv:linters]
 commands =
   flake8 {posargs}