Add support for S3 storage backend

[Connor Chamberlain]

Currently, upstream Glance has S3 backend functionality which the Glance charm does not support.

The goal in this case is to allow Glance to be configured to use an S3 compatible server, such as Amazon S3 or Ceph radosgw, for backend object storage.

https://docs.openstack.org/glance/latest/configuration/configuring.html#configuring-the-s3-storage-backend

[Nobuto Murata (nobuto)]

https://review.opendev.org/c/openstack/charm-glance/+/781748

[Nobuto Murata (nobuto)]

We will take it over and propose an updated patch set on top of the previous work. Thanks!

[Nobuto Murata (nobuto)]

Subscribing ~field-medium for reviewing the patchset in: https://review.opendev.org/c/openstack/charm-glance/+/781748

This feature is required for ongoing customer engagements.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-guide/+/793786

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-guide/+/793786
Committed: https://opendev.org/openstack/charm-guide/commit/96646c657c737db1878287c280a33ff4c459cd8e
Submitter: "Zuul (22348)"
Branch: master

commit 96646c657c737db1878287c280a33ff4c459cd8e
Author: Nobuto Murata <email address hidden>
Date: Mon May 31 21:49:45 2021 +0900

    glance charm: external S3 compatible storage support

    Change-Id: Iaa1f9b643df2b9ab80664064b6cd130bab2bed67
    Closes-Bug: 1919338

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-glance/+/781748
Committed: https://opendev.org/openstack/charm-glance/commit/a6dc972b098587aee1d7b0d6769bb95530ca81e9
Submitter: "Zuul (22348)"
Branch: master

commit a6dc972b098587aee1d7b0d6769bb95530ca81e9
Author: Nobuto Murata <email address hidden>
Date: Mon May 24 10:52:26 2021 +0900

    Allow enabling S3 object storage backend

    S3 backend has been available (again) since Ussuri in the upstream
    Glance. We will enable the backend with config options for an external
    S3 storage information.

    Co-authored-by: Connor Chamberlain <email address hidden>
    Co-authored-by: Vladimir Grevtsev <email address hidden>
    func-test-pr: https://github.com/openstack-charmers/zaza-openstack-tests/pull/574
    Closes-Bug: #1919338
    Change-Id: Id76a74cc6041b9c3364399254681138475f19935