Allows configuring many default settings for certificates,
all of which can still be overridden on a per-cert basis.
Some options have been moved into .defaults from security.acme,
namely email, server, validMinDays and renewInterval. These
changes will not break existing configurations thanks to
mkChangedOptionModule.
With this, it is also now possible to configure DNS-01 with
web servers whose virtualHosts utilise enableACME. The only
requirement is you set `acmeRoot = null` for each vhost.
The test suite has been revamped to cover these additions
and also to generally make it easier to maintain. Test config
for apache and nginx has been fully standardised, and it
is now much easier to add a new web server if it follows
the same configuration patterns as those two. I have also
optimised the use of switch-to-configuration which should
speed up testing.
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