The certificate management chapter explains how to upload a custom
certificate, but not how to go back to the default one. This is
especially needed if a broken certificate made the web interface
unreachable, so that the 'Delete Custom Certificate' button cannot be
used anymore.
Add a section for it, with the button in the web interface and the
'pvenode cert delete --restart 1' command as fallback, which deletes the
custom certificate and key and restarts pveproxy, so that it uses the
default node certificate again.
Signed-off-by: Michal Fox <me@dualfroz.com>