Improve calls to /site/automation

On our servers, it represent more than 50% of HTTP requests. We use it to retrieve all operations possible on the server, parameters included.

The idea is to defer the call of fetch_api():

• only if needed (to validate parameters for instance); but as Drive calls are controled by core developers, parameters check can be salefly removed. We still use it to check few possible operations, investigate if we can use simple variables instead.
• use a property for self.operations to fill it only on demand, and cache the result.

Any other idea?