Comment 4 for bug 1068843

Work items to do all this:

* add celery job that wraps maas-import-pxe-files
* add settings page setting to set proxy url
* add settings page setting to say whether to run maas-import-pxe-files
* amend celery job to take proxy url
* celery job to abort if setting is False
* celerybeat schedule for m-i-p-f

* BootImage schema to reference NodeGroup
* report_boot_images to say which nodegroup it is on
* report_boot_images to run on all celeryds
* change beat schedule to run on all workers