implemented basic ldap auth support

Change-Id: If25b4b597487a41a332b6019b8433688de19bf5c
Reviewed-on: http://review.couchbase.org/44123
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>