Change of default behaviour, retrofit-action fails if latest LTS image not available

[Frode Nordahl]

During the 20.05 cycle we prepared for new LTS by adding the ``retrofit-series`` configuration option.

The default for this configuration option is determined at run time by looking up distro-info data using the ``distro-info`` command to get the latest LTS.

Previously the charm was hard coded to choose Bionic 18.04 images.

Now that Focal 20.04 LTS is out, a side effect of this change is that the retrofit action stops working unless the user has configured their glance-simplestreams-sync charm to also sync focal images.

While it is great to be on the edge and move users over to the mots recent release as quickly as possible this change of default behavior might be problematic.

Do we need a fall back to bionic or at the very least release note the new behavior?

[Frode Nordahl (fnordahl)]

https://review.opendev.org/#/c/727189/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/727189
Committed: https://git.openstack.org/cgit/openstack/charm-octavia-diskimage-retrofit/commit/?id=4dfbf7fdea253440f796236ba8f4d6d1445ee4e5
Submitter: Zuul
Branch: master

commit 4dfbf7fdea253440f796236ba8f4d6d1445ee4e5
Author: Frode Nordahl <email address hidden>
Date: Tue May 12 15:21:45 2020 +0200

    Add fall back for retrofit series

    Default is to use most recent LTS. If no value is configured and
    the charm is not able to find the most recent LTS image in Glance
    it will attempt to find a image matching the series of the unit
    the charm is run on.

    Change-Id: I5143372712d2cd6c37bfffae8aac519a88732659
    Closes-Bug: #1878144