Add etcd support in mesos bay

[hongbin]

There is a bug in mesos for adding support for etcd https://issues.apache.org/jira/browse/MESOS-1806 . Once that bug is fixed, we could consider supporting etcd in mesos bay.

[Daneyon Hansen (danehans)]

To provide additional background, this tech-debt bug was created as part of adding the Flannel network-driver to the Mesos bay type. This effort is in support of implementing the Magnum Container Networking Model [1].

At the time of this writing, Flannel [2] requires Etcd and does not support other key-value stores. Additionally, Mesos only supports Zookeeper as a key-value store. Instead of trying to "force" Flannel into the Mesos bay type, it is my recommendation not to implement the Flannel network-driver for Mesos bay types until [3] is implemented and has time to mature.

[1] https://github.com/openstack/magnum/blob/master/specs/container-networking-model.rst
[2] https://github.com/coreos/flannel
[3] https://issues.apache.org/jira/browse/MESOS-1806