The charm didn't work unless a mandatory bind-rndc interface was
present. However, the dns-slaves option was supposed to provide an
alternative to needing to relate the unit to the (say) designate-bind
charm. This change allows dns-slaves and/or a bind-rndc relation
and configures the underlying service accordingly.
Also some fixups to the tests to simplify handler verification using
more recent charms.openstack features.
Also note that the bind-rndc interface needed a fix (the depends-on)
for if/when a bind-rndc relation is removed; the interface incorrectly
maintained that the relation was present when it was not.
Change-Id: Ib2c883e623292520224f882aef09d3710e1e1348
Closes-Bug: #1693162
Depends-On: I523fecff4e80734772872a8a6d2507f1e2162ae3
fd98fa0722