Docs/chameleon g5k fit

Let's take the hard work done by @aarmange and @drosendo and let's make it become a reality :)

• I've moved this edge use case in the following directory: enoslib/docs/tutorials/multi_providers
• I've adapted the reservation logic to align with the new Providers API. This makes the leases on the Fit + G5k synchronized.
• I also changed some things here and there in the G5k/Fit examples: remove the need to deploy the environment, use an idiomatic way of getting the IPv6 address of the cloud server.

We can iterate here to polish the remaining parts.