Remove need for guacamole.properties

The guacamole.properties file will always be needed for basic configuration information, but certain values should definitely have defaults (like the guacd hostname and port, and the authentication provider), and the default Guacamole configuration should "just work" without any guacamole.properties file at all.

The file should ultimately only be used for special configuration information or overrides.

Eventually, even adding a new extension should not require changing guacamole.properties. The extension itself should contain enough information for Guacamole to:

1. Determine whether the extension is compatible
2. Load and initialize it along with any other installed extensions