Comment 4 for bug 1741723

Reviewed: https://review.openstack.org/538688
Committed: https://git.openstack.org/cgit/openstack/charms.openstack/commit/?id=1ca4bd0ab941f6c7aebf86bdf5ff47fdc1d2d3e2
Submitter: Zuul
Branch: master

commit 1ca4bd0ab941f6c7aebf86bdf5ff47fdc1d2d3e2
Author: Dmitrii Shcherbakov <email address hidden>
Date: Sat Jan 27 22:42:16 2018 +0300

    add string template rendering capability

    In some cases software deliberately allows drop-in config file usage
    capabilities, for example, when it comes to enforcing policy, the
    desired behavior varies from an operator to operator. For that reason it
    is sometimes desirable to supply custom templates via config options.

    Another use-case is templates that are passed from subordinates for a
    primary charm to render.

    Given that properties and desired adapters can be arbitrary the change
    uses a dict of meta tuples of the following format to render templates
    from strings based on adapter properties:

    {config_file_path: (relation_name, adapter property)}

    relation names must be normalized (lowercase, underscores instead of
    dashes. "options" relation name is used for a config adapter as usual.

    In summary a string config file path should be used:

    1. in the restart_map for a given derived class;
    2. in string_templates dict as a key for a meta tuple

    Change-Id: Ic85b22d0e5d497c49c75243e3c280140f940df66
    Closes-Bug: #1741723