improve the doc for building the iso

[Daniele Pizzolli]

Sometimes build iso fails due to the instability of the file system layer.

This is worse in virtual machine enviromnent.

The main issue are shared filesystem with the host and docker.

For docker problems see the partial log of a couple of failed build at:

http://paste.openstack.org/show/782ZFEM4PloD0i1dolig/

To mitigate the issue add documentation about the known workarounds.

[Daniele Pizzolli (daniele-pizzolli)]

Not sure why hudson-openstack does not add the changeset link, so here it is:

https://review.openstack.org/#/c/259484/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/259484
Committed: https://git.openstack.org/cgit/openstack/fuel-web/commit/?id=395d7e7a1d96d6715b907d6e189ead39416a8e43
Submitter: Jenkins
Branch: master

commit 395d7e7a1d96d6715b907d6e189ead39416a8e43
Author: Daniele Pizzolli <email address hidden>
Date: Fri Dec 18 15:56:04 2015 +0100

    Improve the doc for building the iso in a vm

    - more generic doc (address both virtualbox and vagrant)

    - suggest how to use the more reliable docker's aufs storage backend

    Closes-Bug: #1527656
    Change-Id: I825848a8e715f08e3f6ac342d745d4633f87d992