MB-8731: disallow non-utf8 passwords

Because otherwise they cannot be sent over json

Change-Id: I53241a20d3426932f9edd248dd1503939522fa3c
Reviewed-on: http://review.couchbase.org/28306
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>