Tests: Support multiple browsers.
Dashboard: Store machine list permanently.

* The list is managed by cockpitd.
* There is a dedicated websocket to access it.
* The dashboard monitors this connection.
Tests: Use firewall-cmd in cockpit.setup.
Tests: Disable NAT during tests.

But enable it during image creation.
Dashboard: Discover machines via OpenSLP.
Web: Allow management of all machines with a hack.

And not just a single random one.  This is achieved by inspecting the
URL during initialization and selecting the right client for it, and
by reloading when switching to a page for a different client.