sdutil: ability to inject the binary.

Also, read the command line from the image, the user no longer
has to copy it into the service definition.
Comment: Refer to a relevant ticket.
get_plugin() w/o mode to return None.
Move shelf to separate plugin.
Fix copy&paste identifier name error.
Only involve sdutil plugin if explicitly declared.
sdutil: allow changing binary source url.
Fix sdutil building command line.
sdutil: Make sure image is pulled.
Add Makefile for gitreceive image.
Normalize entrypoint to a list as well.
Add new tests for sdutil, image rebuilding.