Add functionality that enables configurable secrets sources, with a
hard-coded `env` that provides the existing environmental variable
support.

- This makes it possible to use dynamic zone config with AutoArpa
- Document what gotchas that entails
- Add tests both with another source (for generating zones to populate) and without (no zones to populate)

Resolved a typo that prevented the auto-arpa configuration options from
loading.

This reverts commit 8b82159ee0.

This confirms that in addition to the recently added support for dotted
subdomains that subdomains that are not dotted are supported.  From
RFC1034 Section 3.5 this would be a <subdomain> that contains a single
<label> without <subdomain> recurring.

The is not a zone between delegated.subzone.unit.tests. and unit.tests.,
however we get a delegated subzone error.  This modifies the tests to
succeed with the added record, however the tests fail as it incorrectly
throws the managed subzone error.

Change the name of delegated.subzone, and the tests will pass cleanly.