Add generic /cmd entrypoint w/ cli support

If you do
  docker devmapper:resize -o Key=Value -o Foo=Bar plugin:op arg1 arg2

This will do a GET http request to /cmd/devmapper/resize with the
arguments and options as keys. This will then try to spawn a job on
the engine called "devmapper:resize" with the arg1, arg2, etc as
arguments and the -o options as environment.

Also, any graph driver implementing the engine.Installer interface
will get installed on the engine when the daemon is created.

This will be used by e.g. the devicemapper to allow driver-specific
operations like pool resizing.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)