Allow specifying --couchbase-root

Change-Id: If0b120f1c4047419afd092988a517195503dd7df
Reviewed-on: http://review.couchbase.org/27742
Reviewed-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>