cert-enroll: certificate checking and enrollment

cert-enroll: certificate checking and enrollment

The cert-enroll script handles the initial enrollment of an X.509
host certificate with a PKI server via the EST or SCEP protocols.

Run as a systemd timer or via a crontab entry the script daily
checks the expiration date of the host certificate. When a given
deadline is reached, the host certificate is automatically renewed
via EST or SCEP re-enrollment based on the possession of the old
private key and the matching certificate.