[3538] Corrected the declaration of the example pool in the kea.conf

Recently the "pool" parameter has been renamed to "pools" and its type
was changed from the string to a map. The kea.conf was not updated to
use the new format for the pool definition. This change corrects it.
[3538] Fixed the pool definitions for DHCPv6 in kea.conf.
[3538] Extended description of the subnet4 as pointed out in the review.
[master] Merge branch 'trac3538'
[master] Added ChangeLog entry for #3538.
[trac3482] Check OpenSSL SHA-2 support in configure
update ChangeLog
[trac3471] cryptolink code cleanup
update ChangeLog