Move logic with fallback to bucket name into lcb_create

We don't have to create 'bucket' member to carry the name of the
bucket. We can do it earlier

Change-Id: Ib7db0efd2bae4dcedff8aee3b51465ee7a495de4
Reviewed-on: http://review.couchbase.org/28700
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>