update-iso.sh corrupts grub.cfg timeout on update to previously updated ISO

[Don Penney]

Brief Description
-----------------
When using update-iso.sh to update a previously updated ISO, the grub.cfg timeout command-line gets corrupted due to a bug in the utility.

Severity
--------
Major: System/Feature is usable but degraded

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

Branch/Pull Time/Commit
-----------------------
master, as of 12/17/2019

Test Activity
-------------
Developer Testing

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/699508
Committed: https://git.openstack.org/cgit/starlingx/utilities/commit/?id=f7886f995007789cea898066e6b9f34918fec967
Submitter: Zuul
Branch: master

commit f7886f995007789cea898066e6b9f34918fec967
Author: Don Penney <email address hidden>
Date: Tue Dec 17 16:53:46 2019 -0500

    Fix update-iso.sh handling of existing timeout in grub.cfg

    Fix a bug where the submenu timeout is corrupted by an incorrect sed
    command.

    In addition, update handling of 0 second timeout to set the timeout in
    grub.cfg to 0.001, to avoid issues on some systems with timeout=0.

    Change-Id: I0683f39eb906da512cacc77bde28f8420a73a001
    Closes-Bug: 1856764
    Signed-off-by: Don Penney <email address hidden>

[Ghada Khalil (gkhalil)]

stx.4.0 / medium priority - issue can be avoided by using the update-iso.sh on an unmodified iso only. Therefore, no plan to cherrypick to r/stx.3.0 at this time