CBSE-1043: Backward compatible with plaintext authentication

Change-Id: I78eb6cc9df7fcf41a8d0885ddf110a150de28418
Reviewed-on: http://review.couchbase.org/34947
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>