Comment 8 for bug 1302772

FYI - in upgrading a previously happy Trusty MAAS test enviro, we can reproduce this issue.

$ sudo apt-get update && sudo apt-get upgrade
...
Setting up maas-cluster-controller (1.5+bzr2227-0ubuntu1) ...
Installing new version of config file /etc/maas/templates/power/sm15k.template ...
 * Restarting web server apache2
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 10.17.17.200. Set the 'ServerName' directive globally to suppress this message
   ...done.
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 42, 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.

* Process never completes, user has to break out.