MB-11567: Allow to only use bucket name and password

When it is called from ns_server, only bucket name and bucket pwd
are passwd because the bucket is created first. As such, we should
bypass bucket creation step and only check bucket status.

Change-Id: I9293ff99d34aeff9c45299cec4d3e76d99d76f92
Reviewed-on: http://review.couchbase.org/41292
Tested-by: Bin Cui <bin.cui@gmail.com>
Reviewed-by: Bin Cui <bin.cui@gmail.com>