agent: syslog: explicitly use image CMD

docker 1.2+ does not implicitly make use of the image CMD if the
ENTRYPOINT has been explicitly set on the container. Therefore, in order
to decorate the ENTRYPOINT, we have to also explicitly copy over the CMD
from the image config to the container config.

https://github.com/docker/docker/pull/6909
https://github.com/docker/docker/issues/7437
Merge pull request #299 from spotify/dano/entrypoint-decoration-fix

agent: syslog: explicitly use image CMD