With RSA 4096 instead of ECDSA.

certbot switched to ECDSA by default about two years ago, following
[recommended practices][1].

We are currently using RSA with 4096 bits, which is extremely slow to
sign. Using ECDSA should give us a nice speedup.

[1]: https://wiki.mozilla.org/Security/Server_Side_TLS#Intermediate_compatibility_.28recommended.29

certbot by default sleep 1-480 seconds before renewing, to avoid all
people renewing at :00. In our case the logic is is unnecessary as
systemd is handling it (RandomizedDelaySec=24h).

service

Some machines use certbot, but don't have nginx so we shouldn't force
the reload here.

Signed-off-by: Florian Pritz <bluewind@xinu.at>

Signed-off-by: Florian Pritz <bluewind@xinu.at>