Proposal: Add etcd plugin to detect the mesos master.

[gradywang]

Currently, the etcd-based master contender/detector module is developing in Mesos community, refer to MESOS-1806 for the details. So in the near future，when we deploy the kubernetes cluster on a high-availability model Mesos cluster, it does not need to run another zookeeper cluster just for Mesos. So I create this ticket to enhance mesos-go to have the ability to detect the Mesos master from etcd cluster. I will submit a pull request for this ticket later,

[gradywang]

Related PR #256