confmon: don't repeat work in set_provider_active

This function does a lot of the stuff _prepare() does. Don't do that.

Change-Id: I3fda5b5cd7c690d62a4440ef70e89d5e11afda9d
Reviewed-on: http://review.couchbase.org/37593
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>