Upgrade from precise to precise SRU maas unconditionally disabled system DHCP

[James Troup]

We have a MAAS installation that was installed with original precise
MAAS (0.1+bzr482+dfsg-0ubuntu1) that was using the system (ISC) dhcpd.

When we upgraded it to the SRU MAAS
(1.2+bzr1373+dfsg-0ubuntu1~12.04.1), it unconditionally¹ disabled the
ISC dhcpd by shipping a '/etc/init/isc-dhcp-server.override' file.

The next time the isc-dhcp-server package was upgraded, our dhcpd
stopped and didn't start back up.

AFAIK (and the docs appear to confirm this), running MAAS with your
own DHCP server is a supported configuration, so it seems like the
upgrade process could be a little more flexible.

--

¹ At least, I can't see anything in the maintainer scripts or debconf
  stuff that would mean this file is ever not installed on upgrade.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Julian Edwards (julian-edwards)]

I think this is purely a packaging bug, so invalidating the maas task that was just added.

[James Troup (elmo)]

FWIW, this just caused another production service outage on our machines.