Comment 25 for bug 1629133

Reviewed: https://review.openstack.org/398012
Committed: https://git.openstack.org/cgit/openstack-dev/devstack/commit/?id=4bfbc291eefd92d8b7885f36275b7ff541e067ab
Submitter: Jenkins
Branch: master

commit 4bfbc291eefd92d8b7885f36275b7ff541e067ab
Author: Kevin Benton <email address hidden>
Date: Tue Nov 15 17:26:05 2016 -0800

    Derive IP ranges from new ADDRS_SAFE_TO_USE vars

    The switch to using subnetpools caused quite a bit of confusion
    because it didn't respect the value of FIXED_RANGE. This caused
    conflicts in the gate with it's default IPv4 value of 10.0.0.0/8.

    This patch does a few things to address the issue:
    * It introduces the IPV4_ADDRS_SAFE_TO_USE and IPV6_ADDRS_SAFE_TO_USE
      values and adjusts all of the FIXED_RANGE and SUBNETPOOL_PREFIX values
      to dervive from them by default.
      * This addresses the concern that was raised about implying that
        SUBNETPOOL_PREFIX and FIXED_RANGE are equivalent when setting
        SUBNETPOOL_PREFIX=FIXED_RANGE by default. Now we have a new value
        for the operator specify a chunk of addresses that are safe to
        use for private networks without implementation implications.
      * Backwards compatibility is maintained by alloing users to override
        override all of these values.
    * The default for IPV4_ADDRS_SAFE_TO_USE uses /22 instead of /24
      * Because we want to be able to use subnetpools for auto allocated
        topologies and we want to be able to have a large chunk of
        instances on each network, we needed a little more breathing room
        in the default v4 network size.
    * SUBNET_POOL_SIZE_V4 default is changed from 24 to 26
      * In conjuction with this change and the one above, the default
        subnetpool will support up to 16 64-address allocations.
      * This should be enough to cover any regular gate scenarios.
      * If someone wants a bigger/smaller subnet, they can ask for that
        in the API request, change this value themselves, or use a different
        network entirely.
    * FIXED_RANGE_V6 defaults to a max prefix of /64 from IPV6_ADDRS_SAFE_TO_USE
      * This avoids the private subnet in the non-subnetpool case from being
        larger than /64 to avoid issues identified in rfc 7421.
      * Users can still explicitly set this value to whatever they want.
        This 'max' behavior is only for the default.
      * This allows IPV6_ADDRS_SAFE_TO_USE to default to a /56, which leaves
        tons of room for v6 subnetpools.

    Closes-Bug: #1629133
    Change-Id: I7b32804d47bec743c0b13e434e6a7958728896ea