* 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
Work items to do all this:
* add celery job that wraps maas-import- pxe-files pxe-files
* add settings page setting to set proxy url
* add settings page setting to say whether to run maas-import-
* 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