BMCWeb policy for HTTPS site identity certificate

[Joseph Reynolds]

This is a followup to the OpenBMC security working group meeting 
discussion on 2020-07-22 
(https://docs.google.com/document/d/1b7x9BaxsfcukQDqbvZsU2ehMq4xoJRQvLxxsDUWmAOI).

Background:
Per [BMCWeb configuration 
policy](https://github.com/openbmc/bmcweb#configuration), BMCWeb 
generates a new HTTPS site identity certificate if a usable one cannot 
be found.  You can upload one via APIs described here: 
https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Certs/README.md#redfish-certificate-support

Problem:
BMCWeb apparently treats certificates that are either expired or not 
valid until a future date as unusable (investigation needed).  And 
BMCWeb deletes unusable certificates.  This can confuse the 
administrator, especially considering the BMC's time-of-day clock may 
not be set as expected.

Proposal:
What certificate management policy should BMCWeb use?  Here is an 
initial proposal:
1. certificate is perfectly good - Use the certificate.
2. certificate is good but expired or not yet valid - Use the 
certificate and log a warning.
3. certificate is missing or bad format or algorithm too old - Use 
another certificate or self-generate a certificate (and log that action).
In no case should BMCWeb should delete any certificate.

Discussion?