Enables variants to override creation and ...

... startup by exposing the method `start_site`.

subiquity/server/server.py

@@ -502,6 +502,9 @@ class SubiquityServer(Application):
             controller.add_routes(app)
         runner = web.AppRunner(app, keepalive_timeout=0xffffffff)
         await runner.setup()
+        await self.start_site(runner)
+
+    async def start_site(self, runner: web.AppRunner):
         site = web.UnixSite(runner, self.opts.socket)
         await site.start()
         # It is intended that a non-root client can connect.