QA: Support master-netdev and default NIC parameters

Requested in issue 337.

The parameter “bridge” was not documented and is therefore silently
replaced with “master-netdev”. A note is added to “qa-sample.json”
describing how comments work.

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