* dns/route53: Allow specifying hosted zone ID
This commit adds support for specifying hosted zone ID via the
environment variable AWS_HOSTED_ZONE_ID. If this is not specified, the
previous discovery process is used.
This is useful in environments where multiple hosted zones for the same
domain name are present in an account.
* dns/route53: Fix up getHostedZoneID method params
Now that getHostedZoneID is a method on the DNSProvider struct, there is
no reason for it to take the Route53 client as a parameter - we can
simply use the reference stored in the struct.
While more than likely never to come up in a real-world situation,
during renewal integration testing a value of 120 seconds has
proven to be too high (the old challenge record has not expired
by the time the new one is created).
