Support a custom base image

[OpenStack Infra]

https://review.openstack.org/313295
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/kolla" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 070bf258357c85a7b4411dc97f75df7d19e9792c
Author: Jeffrey Zhang <[e-mail]>
Date: Fri May 6 17:30:47 2016 +0800

    Support a custom base image

    Add a new option(base-image) to support custom base image
    In default, the base-image is equal to base. When using custom
    image, please use command line like:

        ./tool/build.py --base-image my-image --base centos --base-tag 0.1

    DocImpact

    Closes-Bug: #1573544
    Change-Id: If4b9c57a2b68f6f06d3e981f30b2173064e02193

[Christian Berendt (berendt)]

Jeffrey, please add the missing docs...

[Jeffrey Zhang (jeffrey4l)]

Doc is already added. check http://docs.openstack.org/developer/kolla/image-building.html#using-a-different-base-image