In version 0.21, pylint unified all the disable-* (and enable-*)
directives to disable (resp. enable). This leads to a lot of
DeprecationWarning being emitted even if one uses the recommended
version of pylint (0.21.1, as stated in devnotes.rst).

This commit changes all the disable-msg directives to disable.

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>