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

test/fixtures/deploy_with_extensions.yml

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

test/fixtures/deploy_with_proxy_roles.yml

@@ -0,0 +1,46 @@
+service: app
+image: dhh/app
+servers:
+  web:
+    hosts:
+      - "1.1.1.1"
+      - "1.1.1.2"
+  web2:
+    hosts:
+      - "1.1.1.3"
+      - "1.1.1.4"
+    proxy:
+      response_timeout: 15
+registry:
+  username: user
+  password: pw
+builder:
+  arch: amd64
+
+proxy:
+  response_timeout: 10
+
+accessories:
+  mysql:
+    image: mysql:5.7
+    host: 1.1.1.3
+    port: 3306
+    env:
+      clear:
+        MYSQL_ROOT_HOST: '%'
+      secret:
+        - MYSQL_ROOT_PASSWORD
+    files:
+      - test/fixtures/files/my.cnf:/etc/mysql/my.cnf
+    directories:
+      - data:/var/lib/mysql
+  redis:
+    image: redis:latest
+    roles:
+      - web
+    port: 6379
+    directories:
+      - data:/data
+
+readiness_delay: 0
+deploy_timeout: 6