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
Add an end-to-end test for syslog redirection

1. Make the path to syslog used by SyslogRedirectingContainerDecorator
   configurable via the SYSLOG_REDIRECTOR job environment variable.

2. Use the spotify/syslog-redirector image and test that syslog redirection
   actually works and is sending stdout to the specified endpoint.