Skip audit enrollment if user is in the experimental branch

This addresses an issue which caused a user in the experimental branch to be enrolled as audit mode instead of honor mode when selecting the "Pursue Honor Code Certificate" option on track selection page. @wedaly and @dianakhuang, could you please review this?
Thought: suppose a user registers for two different courses.  The `auto_register` setting will be set to `True` by the first registration... and then will still be `True` in the second flow, even if we don't actually want it in that flow.  So shouldn't we reset `auto_register` to `False` right after doing the enrollment?