Merge lp://staging/~mikemc/maas/packaging-fix-1346538 into lp://staging/~maas-maintainers/maas/packaging

- Ensure that the maas-dns postrm script removes maas-related includes from named.conf.options (LP: 1346538)

- Ensure that the maas-dns postrm script removes maas-related includes from named.conf.options (LP: 1346538)

To reproduce:
1. install maas
2. purge maas
3. remove bind9
3. attempt to reinstall bind9:
you should see :
...
Setting up bind9 (1:9.9.5.dfsg-3) ...
 * Starting domain name service... bind9 [fail]
invoke-rc.d: initscript bind9, action "start" failed.
dpkg: error processing package bind9 (--configure):
 subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
 bind9
E: Sub-process /usr/bin/dpkg returned an error code (1)

/var/log/syslog will look like this:

Oct 20 13:03:39 pliable named[22179]: starting BIND 9.9.5-3-Ubuntu -u bind
Oct 20 13:03:39 pliable named[22179]: built with '--prefix=/usr' '--mandir=/usr/share/man' '--infodir=/usr/share/info' '--sysconfdir=/etc/bind' '--localstatedir=/var' '--enable-threads' '--enable-largefile' '--with-libtool' '--enable-shared' '--enable-static' '--with-openssl=/usr' '--with-gssapi=/usr' '--with-gnu-ld' '--with-geoip=/usr' '--with-atf=no' '--enable-ipv6' '--enable-rrl' '--enable-filter-aaaa' 'CFLAGS=-fno-strict-aliasing -DDIG_SIGCHASE -O2'
Oct 20 13:03:39 pliable named[22179]: ----------------------------------------------------
Oct 20 13:03:39 pliable named[22179]: BIND 9 is maintained by Internet Systems Consortium,
Oct 20 13:03:39 pliable named[22179]: Inc. (ISC), a non-profit 501(c)(3) public-benefit
Oct 20 13:03:39 pliable named[22179]: corporation. Support and training for BIND 9 are
Oct 20 13:03:39 pliable named[22179]: available at https://www.isc.org/support
Oct 20 13:03:39 pliable named[22179]: ----------------------------------------------------
Oct 20 13:03:39 pliable named[22179]: adjusted limit on open files from 4096 to 1048576
Oct 20 13:03:39 pliable named[22179]: found 8 CPUs, using 8 worker threads
Oct 20 13:03:39 pliable named[22179]: using 8 UDP listeners per interface
Oct 20 13:03:39 pliable named[22179]: using up to 4096 sockets
Oct 20 13:03:39 pliable named[22179]: loading configuration from '/etc/bind/named.conf'
Oct 20 13:03:39 pliable named[22179]: /etc/bind/named.conf.options:2: open: /etc/bind/maas/named.conf.options.inside.maas: file not found
Oct 20 13:03:39 pliable named[22179]: loading configuration: file not found
Oct 20 13:03:39 pliable named[22179]: exiting (due to fatal error)

Note that /etc/bind/named.conf.options still contains maas-related includes:

options { directory "/var/cache/bind";
include "/etc/bind/maas/named.conf.options.inside.maas";
auth-nxdomain no;
listen-on-v6 { any; }; };

This branch removes those.