Add support for LE expiration notification e-mails on API cert

[blkeller]

Summary:

This PR adds support for setting an e-mail address in the config file that Let's Encrypt can use to send notification e-mails if the API's certificate is expiring soon. When an account at LE is registered and an e-mail address is provided, LE will use this address for this purpose:
https://letsencrypt.org/docs/expiration-emails/

This is a helpful, optional service that LE offers for free, and the e-mails can be useful for an admin to be alerted to potential certificate renewal issues that might otherwise go unnoticed.

The new config option notification_email is an empty string by default, preserving the previous behavior of not registering any address with LE upon account creation. If the user fills in an e-mail address, this value will be used during account creation with LE.

The value of notification_email is passed through to CertMagic and leverages their existing support for this feature.

Example:

Where the config file includes these entries:

[api]
# possible values: "letsencrypt", "letsencryptstaging", "cert", "none"
tls = "letsencryptstaging"
# only used if tls = "letsencrypt"
acme_cache_dir = "api-certs"
# optional e-mail address to which Let's Encrypt will send expiration notices for the API's cert
notification_email = "[email protected]"

This patch yields the following results:

$ sudo rm -Rf /var/lib/acme-dns/api-certs
$ sudo systemctl start acme-dns
$ journalctl -u acme-dns --no-pager | grep -A2 'certificate maintenance'
Apr 28 18:25:38 ns1 acme-dns[13286]: time="2020-04-28T18:25:38-05:00" level=info msg="2020/04/28 18:25:38 [INFO][cache:0xc00008e5f0] Started certificate maintenance routine"
Apr 28 18:25:38 ns1 acme-dns[13286]: time="2020-04-28T18:25:38-05:00" level=info msg="[INFO] acme: Registering account for [email protected]"
Apr 28 18:25:38 ns1 acme-dns[13286]: time="2020-04-28T18:25:38-05:00" level=info msg="2020/04/28 18:25:38 [INFO][ns1.certs.mydomain.com] Obtain certificate"

$ sudo ls /var/lib/acme-dns/api-certs/acme/acme-staging-v02.api.letsencrypt.org/users
[email protected]

$ sudo cat /var/lib/acme-dns/api-certs/acme/acme-staging-v02.api.letsencrypt.org/users/[email protected]/myuser.json
{
        "Email": "[email protected]",
        "Registration": {
                "body": {
                        "status": "valid",
                        "contact": [
                                "mailto:[email protected]"
                        ]
                },
                "uri": "https://acme-staging-v02.api.letsencrypt.org/acme/acct/12345678"
        }
}

[coveralls]

Coverage remained the same at 90.559% when pulling f89224a on blkeller:notification-email into 19069f5 on joohoi:master.

[joohoi]

LGTM! Sorry it took so long to get around to review this.