Use cluster type connection for cbc-bucket-flush

Although flush command is accessible for bucket type connections,
cbc-bucket-flush doesn't use provided bucket name to connect to,
therefore it will fail if the bucket name isn't "default".

Change-Id: I4751dbd1acecfeda2f5c819730f8daa4df43f3bb
Reviewed-on: http://review.couchbase.org/28755
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>