dcos-cli: configure dcos_url
dcos-cli: configure dcos_url

The dcos_url is used by the `dcos node` subcommand. Note that the `node
ssh` command does not work since it requires that the Mesos DNS API is
exposed on one of the externally accessible nodes in the cluster under
`/mesos_dns`.

    $ dcos node ssh --master
    Error while fetching [http://master.mesos:5050/mesos_dns/v1/hosts/leader.mesos.]: HTTP 404: Not Found

ypg-data/mesos-stack,ypg-data/mesos-stack