purging packages can't remove files because dirs are not empty

I do believe it tries to be careful not to delete other packages' files in the process. Did you see any files left in those locations?

IIRC correctly the not deleted files were pyc files.

I think this is a missing #DEBHELPER# in the cluster's prerm. I'll try and fix it.

This bug was fixed in the package maas - 0.1+bzr1223+dfsg-0ubuntu1

---------------
maas (0.1+bzr1223+dfsg-0ubuntu1) quantal; urgency=low

  * New upstream release. (LP: #1062518)

  [ Julian Edwards ]
  * Split packaging of 'maas' into maas-{region,cluster}-controller
    - debian/control: Update accordingly.
    - debian/*.install: Move files accordingly
    - debian/*.{postinst,postrm,preinst}: Move files accordingly.
  * Ensure isc-dhcp-server is disabled when installing maas-dhcp.
  * Ensure maas-dns creates the maas user before trying to chown files.
  * Make maas-cluster-controller autoconfigure itself when upgrading from the
    old maas package. (LP: #1059416)
  * Add missing prerm file for maas-cluster-controller so that .pyc files
    are cleaned up. (LP: #1059973)

  [ Raphaël Badin ]
  * Install maas_local_celeryconfig.py in /etc/maas and symlink to
    /usr/share/maas.
  * debian/maas.postinst: Create rabbitmq celery user/vhost.
  * debian/maas.postinst: Update BROKER_URL in maas_local_celeryconfig.py.
  * Use maas_local_celeryconfig_cluster.py as the local celery
    configuration file for the cluster worker.
  * debian/maas-region-controller.maas-region-celery.upstart: Add region
    worker upstart script.
  * Rename cluster worker upstart script into
    maas-cluster-controller.maas-clluster-celery.upstart.
  * maas-cluster-controller.maas-celery.upstart: use "celeryconfig_cluster"
    as the Celery config module.
  * debian/maas-common.install: Install celeryconfig_common.py.
  * debian/maas-cluster-controller.install: Install celeryconfig_cluster.py.
  * debian/maas-region-controller.install: Install celeryconfig.py.
  * Split celery config into cluster and region configs.
  * Add region celeryd upstart config.
  * Define CELERY_CONFIG_MODULE in
    maas-cluster-controller.maas-cluster-celery.upstart

  [ Jeroen Vermeulen ]
  * Make maas_local_celery_config.py non-world readable.
  * Make maas_local_celeryconfig_cluster.py non-world readable.
  * Set root:maas ownership of local cluster config only *after*
    the maas user/group have been created

  [ Andres Rodriguez ]
  * debian/maas.postinst:
    - Always restart apache2.
    - Handle upgrades for new upstream release.
    - Handle upgrades for celery rabbitmq worker.
  * Add binary package to install client tool.
    - debian/extras/maas-cli: Add binary.
    - debian/maas-cli.install: Add. Install maascli and apiclient.
    - debian/control: Add binary package.
  * debian/control:
    - Depends on freeipmi-tools instead of ipmitool.
    - Conflicts/Replaces on maas for python-maas-client.
    - Depends on python-netifaces, python-lxml.
  * Add python-maas-client binary package:
    - debian/python-maas-client.install: Add. Install 'apiclient' python module.
    - debian/control: Add package. python-django-maas and maas-cli now
      Depend on it.
  * debian/rules: Install maas-dhcp-server upstart job.
  * debian/maas.postrm: Remove celery worker rabbitmq user and host.
  * debian/extras/99-maas-sudoers: Add for maas-dhcp-server upstart job
    instead of isc-dhcp-server (LP: #1055951)
  * debian/maas-region-controller.postinst: Cleanup upgrade rules.
  * debian/maas-cl...