config_controller should not allow a /29 for a DX mgmt network

[Kristine Bujold]

Brief Description
-----------------
If a /29 network is used for the mgmt network in a DX config_controller the address pool will become exhausted when "system storage-backend-add lvm -s cinder --confirmed" is called.

This is a non-recoverable error, it requires a re-install.

Severity
--------
Minor

Steps to Reproduce
------------------
Use a /29 for MGMT in config_controller and then call:
[wrsroot@controller-0 ~(keystone_admin)]$ system storage-backend-add lvm -s cinder --confirmed
Lvm configuration failed, check node status and retry. If problem persists contact next level of support.

Expected Behavior
------------------
There is not enough addresses in the pool so this is expected behavior.

Actual Behavior
----------------
An error saying "Address pool management has no available addresses" is reported.

Reproducibility
---------------
Reproducible

System Configuration
--------------------
One node system, Two node system, Dedicated storage

[Ghada Khalil (gkhalil)]

stx.2019.03 -- minor issue; plan is to enhance the warning returned to the user

[Kristine Bujold (kbujold)]

The user will be presented with this warning which must be confirmed.

Management subnet [192.168.204.0/24]: 192.168.204.0/29
WARNING: Subnet allows only 8 addresses.
This will not allow you to provision a Cinder LVM or Ceph backend.
Do you want to continue with the current configuration? [Y/n]:

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/608290
Committed: https://git.openstack.org/cgit/openstack/stx-config/commit/?id=7d16a8a9d9e9bf56d1b46e11f1903f2734b0781d
Submitter: Zuul
Branch: master

commit 7d16a8a9d9e9bf56d1b46e11f1903f2734b0781d
Author: Kristine Bujold <email address hidden>
Date: Thu Oct 4 15:43:19 2018 -0400

    Enhance warning for mgmt network configuration

    This fix enhance the warning regarding the size of the address pool
    when configuring the management network. A subnet of /29 will only
    allow 8 addresses. If a LVM/Ceph image backend is added (e.g. system
    storage-backend-add lvm -s cinder –confirmed, system
    storage-backend-add ceph -s cinder,glance –confirmed), the command
    will fail with “Address pool management has no available addresses”.
    The user will now be asked to confirm this configuration.

    Closes-Bug: 1796306

    Change-Id: Ib33efff1e929e9754c241d6c93398be5c4d83db8
    Signed-off-by: Kristine Bujold <email address hidden>