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_primary_web_role_override.yml

@@ -2,12 +2,12 @@ service: app
 image: dhh/app
 servers:
   web_chicago:
-    proxy: true
+    proxy: {}
     hosts:
       - 1.1.1.1
       - 1.1.1.2
   web_tokyo:
-    proxy: true
+    proxy: {}
     hosts:
       - 1.1.1.3
       - 1.1.1.4