don't default to essex when codename resolution fails

[Andrea Ieri]

While doing an OpenStack upgrade last week I ran into a hook error showing the following message:

ERROR unit.swift-proxy/0.juju-log Could not load template from templates/ by dispersion.conf or etc_swift_dispersion.conf.

Digging further revealed that the problem was not really with templates, but rather within the codename resolution function: my installation was not matched to any OpenStack release, and the swift charm code was defaulting to essex.

swift-storage:
lib/swift_storage_utils.py
242: release = get_os_codename_package('swift', fatal=False) or 'essex'

swift-proxy:
lib/swift_utils.py
433: release = get_os_codename_package('swift-proxy', fatal=False) or 'essex'

Leaving the specifics of my installation aside, I don't think this default is helpful given that essex is (very) eol and the charm runs into a hook error when trying to render for it anyway.
I think it would be more useful to consider codename resolution failures fatal and immediately block when they happen.