fix(console): a VNC console stays open while its screen does not change

nginx ends a proxied websocket that sends nothing for 60 seconds, and the
VNC console sends nothing while the VM's screen stays the same: an idle
console, an installer's progress screen, closed after a minute and had to
be connected again. novncd now pings the browser every 30 seconds. The SSH
tunnel of an SSH compute sends a keepalive as often, for a firewall on the
way that ends idle connections.

console/novncd

@@ -165,4 +165,7 @@ if __name__ == "__main__":
         web=False,
         target_host="ignore",
         target_port="ignore",
+        # a ping while the VM's screen stays the same: nginx, and a proxy on
+        # the way, end a websocket that sends nothing for 60 seconds
+        heartbeat=30,
     ).start_server()

console/sshtunnels.py

@@ -51,7 +51,9 @@ def open_tunnel(host, user, target):
     """A socket connected to target on host, and the Tunnel carrying it.
     ssh never asks for anything: a missing key or an unknown host key fails."""
     ours, theirs = socket.socketpair()
-    argv = ["ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=10", "-W", target]
+    # ServerAliveInterval: a console whose screen stays the same sends nothing,
+    # and a firewall on the way may end an idle connection
+    argv = ["ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=10", "-o", "ServerAliveInterval=30", "-W", target]
     if user:
         argv += ["-l", user]
     tunnel = Tunnel(argv + ["--", host], theirs)

console/tests/test_sshtunnels.py

@@ -15,7 +15,7 @@ class SshTunnelTests(TestCase):
             sock, tunnel = sshtunnels.open_tunnel("kvm1.example.com", "root", "127.0.0.1:5900")
         sock.close()
         self.assertEqual(popen.call_args.args[0], [
-            "ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=10", "-W", "127.0.0.1:5900",
+            "ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=10", "-o", "ServerAliveInterval=30", "-W", "127.0.0.1:5900",
             "-l", "root", "--", "kvm1.example.com"])
         self.assertIs(tunnel.process, popen.return_value)