[Docs] Update prepare-build-env.sh and related documentation

[Sergey Kulanov]

Since we have a lot of changes in build iso workflow we need to reflect them in:
- prepare-build-env.sh script[1];
- related documents [2];

[1]. https://github.com/openstack/fuel-main/blob/master/prepare-build-env.sh
[2]. https://docs.fuel-infra.org/fuel-dev/develop/env.html#building-the-fuel-iso

[Ilya Kharin (akscram)]

Sergey, could you specify in the description of this bug report what changes should be done within this work?

[Sergey Kulanov (skulanov)]

@Ilya, I can propose you to test this patch https://review.openstack.org/#/c/221902/ which shows the changes

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/275296

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/221902
Committed: https://git.openstack.org/cgit/openstack/fuel-main/commit/?id=18f2c14a928bf3e92ccd4d868953361bed61614b
Submitter: Jenkins
Branch: master

commit 18f2c14a928bf3e92ccd4d868953361bed61614b
Author: Sergey Kulanov <email address hidden>
Date: Wed Sep 9 21:29:26 2015 +0300

    Update package list required for building ISO

    Remove unused packages for building environment

    Related-bug: #1517958

    Change-Id: Id660806a59eb9aa81499a6de54ba1ed05a0fc363

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/275296
Committed: https://git.openstack.org/cgit/openstack/fuel-web/commit/?id=65fe4a220f0e4e48d3a35d52a50e2d4b0a252942
Submitter: Jenkins
Branch: master

commit 65fe4a220f0e4e48d3a35d52a50e2d4b0a252942
Author: Sergey Kulanov <email address hidden>
Date: Tue Feb 2 18:10:30 2016 +0200

    Update documentation regarding building ISO

    We have a lot of changes that should be reflected in
    building ISO documentation

    Change-Id: I7226d5fb04c8528a09070f917d9feaad6d7199cd
    Related-bug: #1517958