(Of course, there is no standard perl Docker Engine API, so I wrote my
own thin perl library, basically error handling and JSON marshalling
goo.  But had to deal with LWP/http over UNIX socket to Docker, which
is whacky; fortunately, there was a module, but I can confirm that LWP
is a pain when you get off the beaten track.  Anyway, I added a little
client that is useful for debugging.)

Also, I refactored things a bit so there is now an emulabize-image
script that can be run to test emulabization of docker images
separately from vnode creation.

Also, add a specific Docker version of the prepare script.  This
actually gets run in image commit as an ONBUILD instruction.

Also, always copy the image's master passwd files into /etc/emulab
instead of using those installed by client-install.  Those just don't
apply at all here since we have no idea what the base image is; the
uids/gids in the clientside dirs could be a complete mismatch for the
image.