- Use default umask value of 022 to execute ztp plugins

 - User can provide a custom umask octal value string as part of plugin data
   if a specific umask is required.
    {
      "plugin" : {
        "url" : "http://10.1.1.1/plugins/my-plugin",
        "umask" : "177"
      }
    }

 - Firmware plugin has been modified to always use 022 umask when installing
   sonic image

 - Added unit test cases to test user specified umask value