Enable OCSP stapling

ISSUE MIGRATED FROM https://gitlab.com/aegir/hosting_https/issues/26 (see #2934484: Move back issue management to D.o)

See Security Certificate Revocation Awareness: The case for “OCSP Must-Staple” for background.

On Nginx at least, this is simply adding a couple of lines to the site config:

ssl_stapling on;
ssl_stapling_verify on;

For Apache this looks like;

SSLUseStapling on

and 'SSLSessionCache' set to something.

docs: http://httpd.apache.org/docs/trunk/mod/mod_ssl.html#sslusestapling

I assume this would vary per certificate service ... e.g. self-signed would not benefit here ;)