Use universal_newlines with subprocess.check_output

Python 3 will complain when trying to split a byte-string using a string
so this change enables universal_newlines to make the split work in both
python 2 and 3. Also add a py3 tox environment.

Change-Id: I9af74b6c6cc9cfd0c973c85be04791aa6dc0acd4

os_collect_config/config_drive.py

@@ -147,7 +147,7 @@ def all_block_devices():
     '''Run blkid and yield a BlockDevice for all devices.'''
     try:
         cmd = ['blkid', '-o', 'export']
-        out = subprocess.check_output(cmd)
+        out = subprocess.check_output(cmd, universal_newlines=True)
     except Exception as e:
         logger.error('Problem running "%s": %s', ' '.join(cmd), e)
     else:

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = py27,pep8
+envlist = py35,py27,pep8
 
 [testenv]
 usedevelop = True