Add log forwarder backend

Usage: ./swarmd 'logforwarder dockerhostA dockerhostB' stdoutlogger
Attaches to all containers, creates log messages out of them, and forwards to the attached log facility backend.
Expects log facility backend to implement the "Log" verb.

Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)