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
```
Support env varibales with all CAP like MARATHON_ZK.
service start script also supports MARATHON_*
paved document for supporting environment variable to set options.
Documented that all command line options precedes environment variables.
Reflecting comments by Connor Doyle.
Merge pull request #475 from everpeace/feature/read_env_var

Read command line options from MARATHON_*.