Config Jenkins Mesos plugin in config.xml

This commit adds an extremely basic Mesos slave configuration, and
includes a hard-coded Zookeeper address. It will work for my dev
environment only. This will be re-worked along with some
modifications to jenkins-standalone.sh in a future commit.
allow zookeeper paths to be args to the script

This commit allows the ZooKeeper URL for the Jenkins Mesos plugin to be
provided on the path. This ensures the ZK URI can be specified in
Marathon and doesn't necessarily need to be in version control.