Allow running just one test with tox

Now, "tox -e py26 -- test.unit.proxy.test_server" runs just that one
test file.

"tox -e py26" still runs all the tests with py26, just like before.

Change-Id: I40db12dd5e7cc8f9388e29b30447f70d3bfc4b28

tox.ini

@@ -18,7 +18,7 @@ setenv = VIRTUAL_ENV={envdir}
 deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
-commands = nosetests test/unit {posargs}
+commands = nosetests {posargs:test/unit}
 
 [testenv:cover]
 setenv = VIRTUAL_ENV={envdir}