Comment 2 for bug 1793375

Reviewed: https://review.openstack.org/603914
Committed: https://git.openstack.org/cgit/openstack/tripleo-docs/commit/?id=75c2c3c4447134842cefb56f29285e493c2f5966
Submitter: Zuul
Branch: master

commit 75c2c3c4447134842cefb56f29285e493c2f5966
Author: Steve Baker <email address hidden>
Date: Thu Sep 20 08:47:53 2018 +1200

    Make every reference to the prepare command use sudo

    Even though the installing user is added to the docker group, this
    only takes effect after they restart their shell session. So if they
    install the undercloud and run prepare in the same session, the
    prepare call will fail at the docker operations.

    Rather than explain this caveat, document calling prepare with sudo.
    This is already done on line #266, but this change ensures every
    reference to the command includes sudo.

    Change-Id: Ia14b0964dcaefbbf7265b3960315114219fd3c7b
    Closes-Bug: #1793375