Only install hacking in pep8

There is no need to install all test requirements when we only do
pep8 check.

Change-Id: I12ecd682c38ac700cc9bd5395ef5d92e87c4acca

test-requirements.txt

@@ -2,7 +2,6 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 # Hacking already pins down pep8, pyflakes and flake8
-hacking<0.11,>=0.10.0
 coverage>=3.6 # Apache-2.0
 discover # BSD
 fixtures<2.0,>=1.3.1 # Apache-2.0/BSD

tox.ini

@@ -13,6 +13,7 @@ commands =
   python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
+deps = hacking<0.11,>=0.10.0
 commands = flake8
 
 [testenv:cover]