Store SSH host keys in /var/lib/ssh

For some reason, when OverlayFS is mounted at `/etc/ssh`, SELinux
prevents access both `sshd` and `ssh-keygen` access to the files there.
The AVC denials indicate that (some part of) the process is running in
the `mount_t` domain, which is not allowed to read or write `sshd_key_t`
files.

To work around this issue, without granting `mount_t` overly-permissive
access, we now configure the SSH daemon to read host keys from the
persistent data volume directly, instead of "tricking" it with
OverlayFS.  The `ssh-keygen` tool does not read the `HostKey` options
from `sshd_config`, though, so it has to be explicitly instructed to
create keys in this alternate location.  By using a systemd template
unit with `ConditionPathExists`, we avoid regnerating the keys on every
boot, since the `ssh-keygen` command is only run if the file does not
already exist.

overlay/etc/fstab

@@ -1,3 +1 @@
 PARTLABEL=dch-data /var btrfs subvol=var,nosuid,noexec,nodev 0 2
-PARTLABEL=dch-data /run/etc btrfs subvol=etc,nosuid,noexec,nodev 0 0
-overlay /etc/ssh overlay lowerdir=/etc/ssh,upperdir=/run/etc/rw/ssh,workdir=/run/etc/.work,x-systemd.requires=run-etc.mount 0 0