MB-8243 call getCouchBucket() only once inside create method

This fix is to avoid a race condition found while concurrent
threads calling getCouchBucket() method. We will deal with the
root cause of the race condition in Configuration::getString()
in a separate commit.

Change-Id: I1a83f8eec9827bf0e9b34cc323f0a45c681ea102
Reviewed-on: http://review.couchbase.org/26253
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Jin Lim <jin@couchbase.com>