Add ability to select specific tests for py34

For py27 you can simply run specific tests by calling

    tox -e py27 nova.tests.unit.test_something

Add a similiar option for the py34 environment. This will skip the
blacklist of tests that would otherwise be ignored for python3.

Change-Id: I9bf5901afed4abf592e1aaff2f4820b9e3abc671

tools/pretty_tox3.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+TESTRARGS=$1
+
+if [ -z "$TESTRARGS" ]; then
+    ostestr --blacklist_file tests-py3.txt
+else
+    ostestr -r "$TESTRARGS"
+fi

tox.ini

@@ -35,7 +35,7 @@ commands =
 setenv = {[testenv]setenv}
 commands =
   find . -type f -name "*.pyc" -delete
-  ostestr --blacklist_file tests-py3.txt
+  bash tools/pretty_tox3.sh '{posargs}'
 
 [testenv:functional]
 usedevelop = True