agent: Backport bus type option to dbus-json1.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
web: Use 'payload' instead of 'protocol' option.

Because that is the correct spelling for it.  Previously, everything
was using the dbus-json2 payload type by accident.  (This mostly
worked, but not entirely.)

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #616