Fix requirement handling in tox

Move all test dependencies to test-requirements.txt and update
tox.ini to use that and the main requirements file to build the
virtualenv for testing. Remove redundant references to the different
requirements lists in other tox environment stanzas.

Change-Id: I1ff094778369ebfed567ebbb433ef487066f042a

test-requirements.txt

@@ -1,2 +1,5 @@
 Pillow==2.4.0 # MIT
 sphinx>=1.1.2,!=1.2.0,<1.3
+nose
+mock>=1.0
+coverage>=3.6

tox.ini

@@ -2,17 +2,15 @@
 envlist = py26,py27,py32,py33,pypy,pep8,docs
 
 [testenv]
+deps =
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 # tox passes --pre to pip by default, but we don't want that
 install_command = pip install {opts} {packages}
 distribute = False
 commands = nosetests -d --with-coverage --cover-inclusive --cover-package stevedore []
-deps =
-    nose
-    mock
-    coverage
 
 [testenv:venv]
-deps = -r{toxinidir}/test-requirements.txt
 commands = {posargs}
 
 [testenv:pep8]
@@ -20,5 +18,4 @@ deps = flake8
 commands = flake8 stevedore setup.py
 
 [testenv:docs]
-deps = -r{toxinidir}/docs/requirements.txt
 commands = python setup.py build_sphinx