All role specific proxy configuration

By default only the primary role runs the proxy. To disable the proxy
for that role, you can set `proxy: false` under it.

For other roles they default to not running the proxy, but you can
enable it by setting `proxy: true` for the role, or alternatively
setting a proxy configuration.

The proxy configuration will be merged into the root proxy configuration.

test/fixtures/deploy_with_extensions.yml

@@ -1,6 +1,6 @@
 
 x-web: &web
-  proxy: true
+  proxy: {}
 
 service: app
 image: dhh/app