systemd stub resolver

[Alex Crawford]

Beginning with CoreOS 1151.0.0, we began shipping systemd 231, which
introduced a stub resolver into systemd-resolved. We've gotten reports
that it conflicts with certain deployments [1]. This behavior can be
disabled by masking resolved (`systemctl mask systemd-resolved`), but be
aware that entries within /etc/resolv.conf will no longer be updated.
The alternative is to move the machine to the Beta channel and roll back
to a previous release [2] for the time being.

In response, we have proposed an option to disable the stub resolver [3]
while still preserving the rest of systemd-resolved's functionality. We
plan on rolling systemd 231 to the Beta channel only once this change
(or something equivilent) is integrated and the stub resolver is
disabled by default on CoreOS.

Sorry for the trouble.

-Alex

[1]: https://github.com/coreos/bugs/issues/1545
[2]: https://coreos.com/os/docs/latest/manual-rollbacks.html#forcing-a-channel-downgrade
[3]: https://github.com/systemd/systemd/pull/4061