IPs discovery mechanism

We should implement an IP discovery mechanism that will provide us an ability to determine the following 3 IPs per each instance of Hadoop cluster:

* management_ip - it'll be used by Savanna to access Hadoop cluster's instances;
* internal_ip (or it should be named more clearer) - it'll be added to /etc/hosts on nodes and Hadoop will use these ips to make connections between nodes;
* public_ip - it'll be displayed to end-users (for example, for directly accessing Hadoop's management UI).

I propose the following solution for this problem.

We can add several configurations to savanna.conf - lists of networks (in CIDR notation), for example:

    [networks]
    management=$internal
    internal=10.1.0.0/16,10.2.0.0/16
    public=172.18.0.0/16

This networks will be used just to filter IPs.