Allow tox to accept extra parameters for running individual tests

tox.ini

@@ -15,7 +15,7 @@
 envlist = {py27,py36,pypy}-{aws,openstack}
 
 [testenv]
-commands = {envpython} -m coverage run --branch --source=cloudbridge --omit=cloudbridge/cloud/interfaces/* setup.py test
+commands = {envpython} -m coverage run --branch --source=cloudbridge --omit=cloudbridge/cloud/interfaces/* setup.py test {posargs}
 setenv =
     aws: CB_TEST_PROVIDER=aws
     openstack: CB_TEST_PROVIDER=openstack