Support replacing Zookeeper with Consul

Currently Mesos only support Zookeeper for leader election. While Zookeeper has been widely used it is not actively developed and the configuration of Zookeeper is often cumbersome or difficult.

There is already an ongoing MESOS-1806 which would replace Zookeeper with etcd. It would be great if Mesos could support replacing Zookeeper with Consul for its ease of deployment.

While MESOS-3574 proposed Mesos to do its own leader election and failure detection, replacing Zookeeper with Consul as a short term solution will really benefit a lot of existing Mesos users that want to avoid the dependency of Zookeeper deployment.