default service option in config

Like the comment says, this really shouldn't happen, and I noted in asana that following the regular flow, even if the param isn't in the url, it is OK, that data isn't lost, but this PR gives a solution just in case someone goes outside the normal flow.

The service url needs to be the one that would be passed. https://stagingportal.beyondz.org/login/cas for Canvas and https://staging.beyondz.org/users/service for the main site (which can redirect to canvas - I recommend using the main site as a default).