values for OpenStack charm options openstack-origin and source are not validated

[pmatulis]

Values provided for OpenStack charm options openstack-origin and source are not validated. Please consider fixing this as it can save users a lot of time and frustration. At least the syntax should be covered ('cloud:<series>-<openstack>'), but ideally the embedded values should be compatible as well (e.g. the xenial series is incompatible with OpenStack release Victoria so a value of 'cloud:xenial-victoria' should not be allowed).

[ajkavanagh]

It's possible to validate it, although it would need to be done on a charm-by-charm basis to do the verification prior to the value (attempted to) being used. Otherwise, it's likely the charm would just crash. Note that openstack-origin and source can take a range of values as indicated in the add_source() function at: https://github.com/juju/charm-helpers/blob/master/charmhelpers/fetch/ubuntu.py#L614

@pmatulis The add_source() function can already raise SourceConfigError if an invalid source is provided. What would need to happen is to add a function to charm-helpers called something like validate_source() and use this from each charm's config-changed (or equivalent) function to validate the openstack-origin/source (or other config option) and switch the status to Blocked to indicate an invalid config. Thus, I suspect charm bugs also need to be raised.