[tutorial 012] Set a default value

Now we can define the default value of this variable.

If the user doesn't touch this variable's value, the value is "No proxy"

As user intervention is no more required, so the variable change it's default mode too (from "basic" to "standard").

firefox/00-proxy.yml

@@ -2,5 +2,5 @@
 ---
 version: 1.1
 
-proxy_mode:  # Configure Proxy Access to the Internet
+proxy_mode: No proxy  # Configure Proxy Access to the Internet
 ...