Add skeleton of a decorator for API handler mixins

This patch adds a decorator function that generates required methods for
API handler mixins based on the given list of strings represent required
handlers.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
LGTM but what do you think about creating a separate file to share common code between the core plugin and extension.  For example, this method, supported_methods, handle_api_error, MidonetApiException, and MidonetPluginException?  

What worries me about having this method here, is that plugin will depend on the extension, and then the extension will depend on this method defined here.  We could pull these out into a common file.

Alternatively, pull out handle_api_error, MidonetApiException, and MidonetPluginException into a common file to be shared, and move supported_methods in the base extension file.