Enable posargs for the py35 tests

Adding posargs to the py35 tox testenv so that you can run specific
py35 tests rather than all of them each time. For example:

    $ tox -e py35 designate.tests.test_schema

Change-Id: I34c6cf96e4c1beab5774974b0b0e8b75dd8435fe

tox.ini

@@ -15,7 +15,6 @@ whitelist_externals = sh
                       rm
 commands =
   find . -type f -name "*.py[c|o]" -delete
-  sh tools/pretty_tox.sh '{posargs}'
 passenv = http_proxy
           HTTP_PROXY
           https_proxy
@@ -27,9 +26,15 @@ passenv = http_proxy
           OS_STDERR_CAPTURE
           OS_STDOUT_CAPTURE
 
+[testenv:py27]
+commands =
+  {[testenv]commands}
+  sh tools/pretty_tox.sh '{posargs}'
+
 [testenv:py35]
 commands =
-  ostestr --blacklist_file=tests-py3.txt
+  {[testenv]commands}
+  ostestr --blacklist_file=tests-py3.txt '{posargs}'
 
 [testenv:docs]
 commands = rm -rf doc/build