add string cntl for modifying the HTTP bootstrap URI mode

This is currently undocumented

Change-Id: I748207b51a680582e6462d624b1a25b03aa4df6d
Reviewed-on: http://review.couchbase.org/40975
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>