Enable coredumps on hosts

Coredumps are useful when debugging crashes. This PR installs
`systemd-coredump` when prepping a host.

By default, cores will be stored in `/var/lib/systemd/coredump/`. One
can list them by doing `coredumpctl list` and start debugging one using
`coredumpctl gdb [pid]`.

rhizome/host/bin/prep_host.rb

@@ -95,8 +95,9 @@
 r "apt-get -y install qemu-utils mtools acl"
 
 # We need nvme-cli to inspect installed NVMe cards in prod servers when
-# looking into I/O performance issues.
-r "apt-get -y install nvme-cli" if is_prod_env
+# looking into I/O performance issues. systemd-coredump is useful when
+# debugging crashes.
+r "apt-get -y install nvme-cli systemd-coredump" if is_prod_env
 
 SpdkSetup.prep