minimal changeset to get things working on 18.04

@joshdata squashed pull request #1398, removed some comments, and added these notes:

* The old init.d script for the management daemon is replaced with a systemd service.
* A systemd service configuration is added to configure permissions for munin on startup.
* nginx SSL settings are updated because nginx's options and defaults have changed, and we now enable http2.
* Automatic SSHFP record generation is updated to know that 22 is the default SSH daemon port, since it is no longer explicit in sshd_config.
* The dovecot-lucene package is dropped because the Mail-in-a-Box PPA where we built the package has not been updated for Ubuntu 18.04.
* The stock postgrey package is installed instead of the one from our PPA (which we no longer support), which loses the automatic whitelisting of DNSWL.org-whitelisted senders.
* Drop memcached and the status check for memcached, which we used to use with ownCloud long ago but are no longer installing.
* Other minor changes.

Vagrantfile

@@ -2,14 +2,7 @@
 # vi: set ft=ruby :
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "ubuntu14.04"
-  config.vm.box_url = "http://cloud-images.ubuntu.com/vagrant/trusty/current/trusty-server-cloudimg-amd64-vagrant-disk1.box"
-
-  if Vagrant.has_plugin?("vagrant-cachier")
-    # Configure cached packages to be shared between instances of the same base box.
-    # More info on http://fgrehm.viewdocs.io/vagrant-cachier/usage
-    config.cache.scope = :box
-  end
+  config.vm.box = "ubuntu/bionic64"
 
   # Network config: Since it's a mail server, the machine must be connected
   # to the public web. However, we currently don't want to expose SSH since