Parse DNSKEY into a dnspython type in isctest.kasp.Key.dnskey

Parse DNSKEY into a dnspython type in isctest.kasp.Key.dnskey

Previously, a DNSKEY string from keyfile was returned. This made the
function brittle for further processing, as the string would have to be
split up, concatenated, and TTL could be missing, making string indices
context-dependent.

Parse the DNSKEY rrset into a proper dnspython object and return it.
This makes the output more predictable and reliable, as all the
neccessary parsing is done by dnspython.

(cherry picked from commit 0bf20f8d68705d2b4fa52a0c77a28ec25ab277af)