Add Zookeeper support for discovery package

[sayden]

The app actually uses ETCD as discovery service. Docker Swarm can be configured to use Etcd, Consul and Zookeeper.

It will be great to have support for Zookeeper installations.

Discovery package has an interface for discovery clients in the file client.go. This interface is defined as follows:

type Node struct {
IP string
Containers []map[string]interface{}
Images []map[string]interface{}
}

type InfoService interface {
ListHosts() ([]Node, error)
WatchHosts()
}
ListHosts must return an array of Node connected to the discovery service (Zookeeper) with IP string filled. In Etcd this information is stored under /docker/swarm/nodes
WatchHosts is still wip in Etcd but ideally it must trigger an action in server when a new Node is connected or disconnected to the discovery service. A very direct approach could be to pass to WatchHosts an instance of the websocket connection so that it sends the information to front.