Update edxapp ansible role for third party authorization

I updated edxapp ansible role to include the ability to enable third party authorization via the server-vars.yml file.  A sample of the server-vars.yml file is as follows:

    # Third Party Auth Additions
    EDXAPP_ENABLE_THIRD_PARTY_AUTH: true
    EDXAPP_THIRD_PARTY_AUTH: {
      'Google': {
        "SOCIAL_AUTH_GOOGLE_OAUTH2_KEY": "[GOOGLE-KEY]",
        "SOCIAL_AUTH_GOOGLE_OAUTH2_SECRET": "[GOOGLE-SECRET]"
      }
    }

Where the key ("Client ID" on Google's site) and secret are your own secret and key from [Google's Developer Console](https://console.developers.google.com/).