web: Handle task-less machine setup.

We can't skip the 'action' tab anymore when there are no tasks since
actually adding the machine happens during that step now.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #626
test: Use 'payload' instead of 'protocol' in ph_dbus_ready.

So that we wait for the right client to be ready.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #629
web: Fix visibility on Docker images page

Previously all buttons on that page would be shown/hidden when the
image 'waiting' status changed not just the buttons for the image
itself.  That used to be OK, but now we also have buttons for
containers which should not be affected.

Reviewed-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Closes #627