Properly configure SSH from scratch

dpkg-reconfigure clearly is designed for interactive use. Cloud images
have SSH preinstalled and keys are generated by cloud-init, but
Subiquity installs the `openssh-server` package if selected in the
installer. This seems like reasonable behavior to emulate in a world
where we aren't triggering cloud-init behavior.

See https://github.com/canonical/subiquity/blob/1c4c99562ef678da5fe06cd810a4268d9f63fda4/subiquity/models/ssh.py#L34

build.sh

@@ -53,13 +53,14 @@ EOF
 # Generate SSH keys on first boot
 cat > $mnt/etc/systemd/system/ssh-hostkey-generate.service <<EOF
 [Unit]
-Description=Generate SSH keys on first container boot
+Description=Properly configure OpenSSH keys on first container boot
 Before=ssh.service
 ConditionPathExists=!/etc/ssh/ssh_host_rsa_key
 
 [Service]
 Type=oneshot
-ExecStart=dpkg-reconfigure --default-priority --frontend=noninteractive openssh-server
+Environment=DEBIAN_FRONTEND=noninteractive
+ExecStart=apt-get install --reinstall -qq -y openssh-server
 
 [Install]
 WantedBy=ssh.service