sshd: the daemon it owns starts at boot

The module said the service must be running and nothing about boot, so the
machine's own way back in was enabled only because something before the mesh
had enabled it. All four machines happen to be enabled today; none of them is
enabled because the mesh says so, and a machine adopted tomorrow would run ssh
until its first reboot.

Not `state: running` alone for the same reason the module exists: this is the
one daemon whose absence cannot be fixed remotely.

modules/sshd/module.json

@@ -31,6 +31,7 @@
       "type": "service",
       "unit": "sshd.service",
       "state": "running",
+      "boot": "enabled",
       "restart-on": [
         "config"
       ]