docs: update instructions for TLS cert generation

docs: update instructions for TLS cert generation

Currently we only describe setting the CN field for server certs. This leads
to inevitable pain for users who set it to the fully qualified hostname and
then use a unqualified hostname or IP address to connect in the URI. Describe
the usage of Subject Alt Name extensions, to provide multiple hostnames and
IP addresses. This will help users avoid the classic mistake and is important
future proofing, since at least in browsers, TLS libraries no longer use the
CN field for validation, mandating use of SAN info instead.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>