maas-import-pxe-files fails when run from the command line

Running maas-import-pxe-files from the commandline results in an immediate error:

graham@angua: maas$ bin/py scripts/maas-import-pxe-files --config-file etc/maas/bootresources.yaml
2014-05-23 13:39:52,892 ERROR Unhandled exception; unable to continue.
Traceback (most recent call last):
  File "scripts/maas-import-pxe-files", line 31, in <module>
    main(args)
  File "/home/graham/workspace/maas-work/maas/src/provisioningserver/import_images/boot_resources.py", line 261, in main
    if args.config:
AttributeError: 'Namespace' object has no attribute 'config'

This is because the script is looking for a "config" blob in the arguments to main, but none exists. Nor is it specified that it *could* exist. This looks like a combination of tests not testing the script from end-to-end (nothing tests scripts/maas-import-pxe-files directly; the highest level that tests get to is main()), a piece of missed coding and a test that mocks the config setup so that an appropriate failure never gets raised.

The script doesn't actually need to accept the config blob anymore – that code is a vestigial thinko from the first work to remove bootresources.yaml. It's easiest to just remove that check for args.config and the associated tests.