grid/utils: minor cleanups

 - Use dict.items() to iterate on dict, instead of dict.keys() then dict[key]
 - don't use six.iterkeys(), this dict is small, so we don't need iterkeys()
   on python2 and can already write python3 version
 - don't % string when invoking logger, logging framework do it for us and do
   it only if message will be logged.

slapos/grid/utils.py

@@ -40,7 +40,6 @@ import logging
 import psutil
 import time
 
-import six
 
 from slapos.grid.exception import BuildoutFailedError, WrongPermissionError
 
@@ -150,10 +149,10 @@ def getCleanEnvironment(logger, home_path='/tmp'):
     if old is not None:
       removed_env.append(k)
   changed_env['HOME'] = env['HOME'] = home_path
-  for k in sorted(six.iterkeys(changed_env)):
-    logger.debug('Overridden %s = %r' % (k, changed_env[k]))
+  for k, v in sorted(changed_env.items()):
+    logger.debug('Overridden %s = %r', k, v)
   if removed_env:
-    logger.debug('Removed from environment: %s' % ', '.join(sorted(removed_env)))
+    logger.debug('Removed from environment: %s',  ', '.join(sorted(removed_env)))
   return env