[2.1] Deleting all boot sources and creating a new boot source, does not update the cache

[Blake Rouse]

This is a really bad check:

def is_first_run(sender, instance, *args, **kwargs):
    """Store whether or not this is the first BootSource added to MAAS.

    On first run the ImportResourceService sets the default BootSource then
    caches the stream's contents as normal. Setting the default BootSource
    triggers this signal. This prevents updating the cache twice.
    """
    instance.__is_first_run = BootSource.objects.count() == 0

This makes it so if you deleted all boot sources then create a new source the cache will not be updated. This prevents the ability to make any selections as MAAS thinks the source has no items. This is only an issue with the API and something I am running into with meta-MAAS.