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
@drewcsillag Not sure about the rename. `containerConfig` is already used by the parameter with the same name, which actually holds the config of the container we're about to start. `imageConfig` is the config of the image used by the container.

@davidxia dockerd seems to provide the inherited/effective `CMD` in `imageInfo.config`. API docs are not very informative on this point. Might be worth investigating a bit more.