Read command line options from MARATHON_*.

Read command line options from MARATHON_* environment variables like meos-master/slave.

This could be very useful when you start marathon with docker.  If we had a docker container in which marathon was installed , we can start marathon like this.

```
$ docker run \
    -e "MARATHON_master=zk://localhost:2181/mesos" \ 
    -e "MARATHON_zk=zk://localhost:2181/marathon" \
    marathon /marathon-0.6.1/bin/start
```
(Or, If Dockerfile contains `ENTRYPOINT ["/marathon-0.6.1/bin/start"]`, we can omit the last command in above.)

However, I'm not shell script expart, so I would be pleasure to have feedbacks.