Add additional config cache tests

- Ensure bootstrap callback is invoked for config cache
- Ensure config cache is settable via lcb_cntl_string

Change-Id: I6c8672df4e9975c57776685e60477ee05ae3d5de
Reviewed-on: http://review.couchbase.org/39931
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>