Event.create api needs to provide a sensible default (in the BAO create function) for role id

When creating participants through the api if no role is specified the event role default should be used - at the moment no role default is used. Unfortunately role MUST be used if event capacity & event full is to be calculated correctly. Correct fix should go in the BAO not the api (& potentially instances of setting the default in the form should be removed).

I'm going to work around this for now - hence not putting to 4.4 + 4.6