Accomodate suggestions from last pull request

Hi there, the following changes are for accomodating the suggestions of the last pull request about StudentExerciseTest. In summary, the changes in here are:

1. Fix ChangeLocalUserPassword to try the new password on the browser
2. remove the db_ prefix from create_(user|facility)
3. fix the exercise url to use self.live_server_url
4. have browser_insert_answer return the current number of points