ntp dhclient exit hook does not remove most servers

[Gavin Panella]

/etc/dhcp/dhclient-exit-hooks.d/ntp uses the following command to copy
/etc/ntp.conf, stripping all-but-local NTP servers:

  sed -r -e '/^ *(server *[^1][^2][^7]\.|peer).*$/d' $NTP_CONF

However this expression is almost completely broken. It will eliminate
"server foo.example.com" but not "server foobar.example.com", "server
216.1.2.3" but not "server 217.1.2.3" or "server 10.1.2.3".

This has been noted in bug 575458 by Jeffrey Hutzelman and Robie Basak
has corrected this problem in Xenial, but this behaviour MAY cause
problems for MAAS 2.1 (currently in development) when Trusty is deployed
or used on a device on a MAAS-managed network.

Context: MAAS 2.1 will be managing an NTP service and informing DHCP
clients to use it via the ntp-servers option.

I estimate that this would be a low-impact problem; failure modes are
likely to be fairly benign. I'm filing this bug because, well, it is a
bug, and it's documenting what might turn out to be a genuine problem.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ntp 1:4.2.6.p5+dfsg-3ubuntu2.14.04.8
ProcVersionSignature: Ubuntu 4.2.0-42.49~14.04.1-generic 4.2.8-ckt12
Uname: Linux 4.2.0-42-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.21
Architecture: amd64
Date: Mon Jul 18 11:24:15 2016
InstallationDate: Installed on 2016-04-13 (95 days ago)
InstallationMedia: Ubuntu-Server 14.04.4 LTS "Trusty Tahr" - Release amd64 (20160217.1)
SourcePackage: ntp
UpgradeStatus: No upgrade log present (probably fresh install)