I killed all that and then retried like so, and got the same failure:
ubuntu@critical-maas:~$ sudo apt-get dist-upgrade
[sudo] password for ubuntu: 
E: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem. 
ubuntu@critical-maas:~$ sudo dpkg --configure -a
Setting up uvtool (0~bzr92-0ubuntu1) ...
Setting up python-sip (4.15.5-1build1) ...
Setting up amtterm (1.3-1ubuntu1) ...
Processing triggers for initramfs-tools (0.103ubuntu4) ...
update-initramfs: Generating /boot/initrd.img-3.13.0-22-generic
Setting up python-iscpy (1.05-0ubuntu2) ...
Setting up pulseaudio-utils (1:4.0-0ubuntu11) ...
Setting up xserver-xorg-video-sis (1:0.10.7-0ubuntu6) ...
Setting up python-pil (2.3.0-1ubuntu3) ...
Setting up usb-creator-common (0.2.56) ...
Setting up python-qt4-dbus (4.10.4+dfsg-1build1) ...
Setting up ssh-askpass-gnome (1:6.6p1-2) ...
Setting up ubuntu-artwork (1:14.04+14.04.20140402-0ubuntu1) ...
Setting up indicator-sound (12.10.2+14.04.20140401-0ubuntu1) ...
Installing new version of config file /etc/xdg/autostart/indicator-sound.desktop ...
Setting up cloud-image-utils (0.27-0ubuntu8) ...
Setting up python-smbc (1.0.14.1-0ubuntu2) ...
Setting up python3-lxml (3.3.3-1) ...
Setting up unattended-upgrades (0.82.1ubuntu2) ...
Setting up python3-markupsafe (0.18-1build2) ...
Setting up python-cups (1.9.66-0ubuntu2) ...
Processing triggers for ureadahead (0.100.0-16) ...
Setting up curtin-common (0.1.0~bzr124-0ubuntu1) ...
Setting up python3-cairo (1.10.0+dfsg-3ubuntu2) ...
Setting up juju-core (1.17.7-0ubuntu1) ...
update-alternatives: using /usr/lib/juju-1.17.7/bin/juju to provide /usr/bin/juju (juju) in auto mode
Setting up python3-simplestreams (0.1.0~bzr341-0ubuntu1) ...
Setting up python-pam (0.4.2-13.1ubuntu3) ...
Setting up unity-scope-texdoc (0.1+14.04.20140328-0ubuntu1) ...
Setting up lightdm (1.9.14-0ubuntu2) ...
Installing new version of config file /etc/apparmor.d/abstractions/lightdm ...
Setting up python-qt4 (4.10.4+dfsg-1build1) ...
Setting up python-markupsafe (0.18-1build2) ...
Setting up shotwell-common (0.18.0-0ubuntu4) ...
Setting up python-imaging (2.3.0-1ubuntu3) ...
Processing triggers for libc-bin (2.19-0ubuntu3) ...
Setting up python3-magic (1:5.14-2ubuntu3) ...
Setting up python3-software-properties (0.92.35) ...
Setting up unity-scope-calculator (0.1+14.04.20140328-0ubuntu1) ...
Setting up mobile-broadband-provider-info (20140317-1) ...
Setting up uvtool-libvirt (0~bzr92-0ubuntu1) ...
Pool uvtool marked as autostarted
Setting up onboard (1.0.0-0ubuntu3) ...
Setting up python-commandnotfound (0.3ubuntu12) ...
Setting up python3-yaml (3.10-4build4) ...
Setting up maas-cluster-controller (1.5+bzr2204-0ubuntu1) ...
 * Restarting web server apache2                                        AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message
                                                                 [ OK ]
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/usr/lib/python2.7/dist-packages/provisioningserver/__main__.py", line 43, in <module>
    main()
  File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/__init__.py", line 592, in __call__
    self.execute(argv)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/__init__.py", line 587, in execute
    args.handler.run(args)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/upgrade_cluster.py", line 210, in run
    hook()
  File "/usr/lib/python2.7/dist-packages/provisioningserver/upgrade_cluster.py", line 185, in generate_boot_resources_config
    rewrite_boot_resources_config(config_file)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/upgrade_cluster.py", line 168, in rewrite_boot_resources_config
    tftproot = Config.load_from_cache()['tftp']['root']
  File "/usr/lib/python2.7/dist-packages/provisioningserver/config.py", line 240, in load_from_cache
    cls._cache[filename] = cls.parse(stream)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/config.py", line 183, in parse
    return cls.to_python(yaml.safe_load(stream))
  File "/usr/lib/python2.7/dist-packages/formencode/api.py", line 439, in to_python
    value = tp(value, state)
  File "/usr/lib/python2.7/dist-packages/formencode/schema.py", line 161, in _to_python
    value_dict, state)
formencode.api.Invalid: The input field 'boot' was not expected.