Make sample config just have the HS config

The other options are things people usually won't need or want to
set, so just keep it to the basics.

 * Rename config sample to match element-web
 * Update / simplify build instructions
 * Rename nginx config file to differentiate it from EC config

config/config.sample.json

@@ -0,0 +1,8 @@
+{
+  "default_server_config": {
+      "m.homeserver": {
+          "base_url": "https://call.ems.host",
+          "server_name": "call.ems.host"
+      }
+  }
+}