Fix new pylint errors

Under squeeze pylint reports the following errors:
************* Module ganeti.serializer
E1103:155:LoadSignedJson: Instance of 'False' has no 'get' member (but some types could not be inferred)
************* Module ganeti-masterd
E1103:166:ClientRqHandler.handle: Instance of 'False' has no 'get' member (but some types could not be inferred)
E1103:167:ClientRqHandler.handle: Instance of 'False' has no 'get' member (but some types could not be inferred)
************* Module gnt-instance
E1103:431:BatchCreate: Instance of 'False' has no 'keys' member (but some types could not be inferred)

For the first two cases it's actually wrong: we had checked before that
the variable on which "get" is called is actually a dict. In the third
case though such check doesn't exist, so we add it. Then we silence the
error all three times.

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