Start multi-user.target on deploy. Fixes NixOS#1063.

This makes `nixops deploy` congruent (the declared services
will always be running after deploy, no matter what the state
of the server was before the deploy).

nixops/deployment.py

@@ -740,6 +740,11 @@ def worker(m):
                     # FIXME: should check which systemd services
                     # failed to start after the reboot.
 
+                if res == 0:
+                    res = m.run_command("systemctl isolate multi-user.target")
+                    if res != 0:
+                        raise Exception("unable to start multi-user.target")
+
                 if res == 0:
                     m.success("activation finished successfully")