r/serterm: Deploy serial terminal multiplexer

Using `tmux`, we can spawn a bunch of `picocom` processes for the serial
ports connected to other server's console ports.  The
_serial-terminal-server_ service manages the `tmux` server process,
while the individual _serial-terminal-server-window@.service_ units
create a window in the `tmux` session.

The serial terminal server runs as a dedicated user.  The SSH server is
configured to force this user to connect to the `tmux` session.  This
should help ensure the serial consoles are accessible, even if the
Active Directory server is unavailable.

roles/serterm/handlers/main.yml

@@ -0,0 +1,9 @@
+- name: restart serial-terminal-server
+  service:
+    name: serial-terminal-server
+    state: restarted
+
+- name: reload sshd
+  service:
+    name: sshd
+    state: reloaded