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/ssh/sshd_config

@@ -0,0 +1,16 @@
+PasswordAuthentication no
+
+UsePAM yes
+PrintMotd no
+PrintLastLog no
+
+Subsystem	sftp	/usr/lib64/misc/sftp-server
+
+AcceptEnv LANG LC_ALL LC_COLLATE LC_CTYPE LC_MESSAGES LC_MONETARY LC_NUMERIC LC_TIME LANGUAGE LC_ADDRESS LC_IDENTIFICATION LC_MEASUREMENT LC_NAME LC_PAPER LC_TELEPHONE
+AcceptEnv COLORTERM
+
+HostKey /var/lib/ssh/ssh_host_rsa_key
+HostKey /var/lib/ssh/ssh_host_ecdsa_key
+HostKey /var/lib/ssh/ssh_host_ed25519_key
+
+Include /etc/ssh/sshd_config.d/*.conf