Run pylint over QA code too

Right now, the QA code is not covered by pylint, and this shows at
least one low-impact bug.

This patch does the necessary changes to make QA pylint-clean, and the
changes the makefile to run pylint for it.

Notable changes:

- qa_utils.GenericQueryTest: randfields was not used at all, and my
  belief is that it was indented to be used in order not to modify the
  input list; so I replaced randfields with fields, so we only shuffle
  the our local copy
- qa_node.TestOutOfBand was using it's own copy of AcquireNode(), so I
  replaced it with the existing version
- qa_os: was using 'dir' in a couple of places, replaced with dirname

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>