use syslog as the default log driver for rock-ons.

[schakrava]

Currently, the default json logging is used which is evidently meant for development purposes. With docker 1.6, there is syslog option which will send logs to the host's syslog. We should use this so the Rock-on root Share doesn't get filled up by applications logging a lot.

Another options is to completely disable logging by using the "none" driver, but this is probably not the right thing to do given Rock-ons are fairly new. We should provide this as an option in the future, however.

Here are some useful links:
https://blog.docker.com/2015/04/docker-release-1-6/
https://www.sumologic.com/2015/04/16/new-docker-logging-drivers/