Fix pylint 'E' (error) codes

This patch adds some silences and tweaks the code slightly so that
“pylint --rcfile pylintrc -e ganeti” doesn't give any errors.

The biggest change is in jqueue.py, the move of _RequireOpenQueue out of
the JobQueue class. Since that is actually a function and not a method
(never used as such) this makes sense, and also silences two pylint
errors.

Another real code change is in utils.py, where FieldSet.Matches will
return None instead of False for failure; this still works with the way
this class/method is used, and makes more sense (it resembles more
closely the re.match return values).

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