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)