Add gateways everywhere when new node group is added

https://review.openstack.org/258479
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit 469f5629a196122d7f3ee82e3e4c778439d48e24
Author: Aleksey Kasatkin <email address hidden>
Date: Tue Dec 15 19:28:36 2015 +0200

    Add gateways everywhere when new node group is added

    It is required for environments with multiple node groups.
    Before, user should set gateways for all networks via API.
    Otherwise, he had got an error.

    Now, it is done automatically on node group creation:
    1. Set gateways for all networks in new node group.
    2. Set gateways for networks in default node group where they were not set.

    IP ranges of networks of default node group are cut or deleted if they
    intersect with new gateways. IP addresses which intersect with new gateways
    are deleted as well. Need to add appropriate message on UI when new node
    group is being added.

    DocImpact
    Closes-Bug: #1517400
    Partial-Bug: #1484008

    Change-Id: I6580d97b849ccd991bbfb65308aa35f217f30987