Update class Key

Update class Key

Because we want to check the metadata in all three files, a new
value in the Key class is added: 'privatefile'. The 'get_metadata'
function is adapted so that we can also check metadata in other files.

Introduce methods to easily retrieve the TTL and public DNSKEY record
from the keyfile.

When checking if the CDS is equal to the expected value, use the DNSKEY
TTL instead of hardcoded 3600.

(cherry picked from commit 97f6b7ad11c3c1d6c1ceeaada5776759649afa49)