Move test requirement coverage into tox.ini

Like commented in the test-requirements.ini file the test requirement
coverage can be moved into the tox.ini file when tox>=1.4 is required.
The minimum version at the moment is 1.6.

Change-Id: Ic71f63fb0fab0470e4732099bb8ab5c13dd98538

test-requirements.txt

@@ -7,10 +7,5 @@ testrepository>=0.0.17
 testscenarios>=0.4
 testtools>=0.9.32
 
-# when we can require tox>= 1.4, this can go into tox.ini:
-#  [testenv:cover]
-#  deps = {[testenv]deps} coverage
-coverage>=3.6
-
 # mocking framework
 mock>=1.0

tox.ini

@@ -15,10 +15,13 @@ commands = python setup.py testr --slowest --testr-args='{posargs}'
 commands = flake8
 
 [testenv:cover]
+deps = {[testenv]deps}
+    coverage
 setenv = VIRTUAL_ENV={envdir}
 commands =
   python setup.py testr --coverage
 
+
 [testenv:venv]
 commands = {posargs}