Unclear type of key needed on 'key' charm setting

[David Negreira]

When one goes through the charm config.yaml, it is unclear the type of key necessary for the 'key' setting, one has to go through the source code to find which type of key is supported.
This bug is raised in order to bring more visibility to users who want to know what type of key is supported.
As per `hooks/charmhelpers/fetch/ubuntu.py` currently:

def import_key(key):
"""Import an ASCII Armor key.

    A Radix64 format keyid is also supported for backwards
    compatibility. In this case Ubuntu keyserver will be
    queried for a key via HTTPS by its keyid. This method
    is less preferrable because https proxy servers may
    require traffic decryption which is equivalent to a
    man-in-the-middle attack (a proxy server impersonates
    keyserver TLS certificates and has to be explicitly
    trusted by the system).

    :param key: A GPG key in ASCII armor format,
                  including BEGIN and END markers or a keyid.
    :type key: (bytes, str)
    :raises: GPGKeyError if the key could not be imported
    """

I believe the 'param' describes what we want to put on config.yaml for better visibility.

[Felipe Reyes (freyes)]

Reactive charms will carry this description provided by the 'apt' layer - https://git.launchpad.net/layer-apt/tree/config.yaml#n22

[Alex Kavanagh (ajkavanagh)]

> Reactive charms will carry this description provided by the 'apt' layer - https://git.launchpad.net/layer-apt/tree/config.yaml#n22

Note for OpenStack reactive charms: they don't use this layer as they use the charms.openstack library instead which uses layer-openstack and layer-openstack-* layers which provide the apt functionality from charm-helpers.

The ceph-osd charm is a 'classic' charm, in that it doesn't use the reactive framework, and uses charm-helpers 'vendored-in' to the charm via the charm_helpers_sync.py utility (see the Makefile in classic OpenStack charms).

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-ceph-osd/+/807311
Committed: https://opendev.org/openstack/charm-ceph-osd/commit/13cc2411e3130726815ef1f94f26763459c450b1
Submitter: "Zuul (22348)"
Branch: master

commit 13cc2411e3130726815ef1f94f26763459c450b1
Author: David Negreira <email address hidden>
Date: Fri Sep 3 14:41:30 2021 +0200

    Add accepted formats on 'key' configuration

    Add a bit more information on config.yaml about the type of keys
    that can be passed as a parameter to the 'key' configuration.

    Signed-off-by: David Negreira <email address hidden>
    Change-Id: Ieeb0f598ca9a7188f81619c2b4fe88af14f260fd
    Closes-Bug: #1942605