configure_source helper does not allow adding arbritrary sources

[Frode Nordahl]

A charm may be in need of adding multiple sources (for example vendor specific PPAs), and it would be useful if the ``configure_source`` method would allow to do this.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/725261

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/725261
Committed: https://git.openstack.org/cgit/openstack/charms.openstack/commit/?id=21da52526c7f489ec9483fa817f76e106d61967d
Submitter: Zuul
Branch: master

commit 21da52526c7f489ec9483fa817f76e106d61967d
Author: Frode Nordahl <email address hidden>
Date: Mon May 4 14:48:08 2020 +0200

    Allow specifying config key in call to configure_source

    A charm may be in need of adding multiple sources (for
    example vendor specific PPAs), and it would be useful
    if the ``configure_source`` method would allow to do
    this.

    In such a use case overriding the ``source_config_key``
    class variable would not be practical, so we allow
    to optionally specify it as a argument to the method.

    Change-Id: Ia5a8f05e863251520771a1c01a5f7dddf4be34d0
    Closes-Bug: #1876720