Merge branch 'devel-2.1'

* devel-2.1:
  Add "adopt" to the allowed disk parameters
  Improve pylintrc for pylint 0.21+
  Fix warnings with Python 2.6
  Fix a small bug introduced in cf26a87a
  Fix the type of 'valid' attribute in LUDiagnoseOS

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>