[cwctl] rewrite wsgi method choices
This will help devs to add new choices by adding their callback to WSGI_CHOICES.
Note:
``options`` becomes a property because ``wsgichoices()`` should be
evaluated at the last-minute.
from datetime import datetime
for rtype in ('cw_support', 'cw_dont_cross', 'cw_may_cross'):
drop_relation_type(rtype)
add_entity_type('CWSourceSchemaConfig')
if not 'url' in schema['CWSource'].subjrels:
add_attribute('CWSource', 'url')
add_attribute('CWSource', 'parser')
add_attribute('CWSource', 'latest_retrieval')