Review cache management in enos_g5k

We're facing some issue with the cache (based on ring: https://ring-cache.readthedocs.io/en/stable/)

• First this appears to not being process safe when using on NFS (...The doc states it should be...)
• Second credentials may leaks in the cached objects

Proposals:

• remove the cache (will but some load on g5K api)
• don't change anything except we can do in-memory caching to not leak data on the fs but still benefit from the cache (use ring.lru() for that)
• make it optional (so that it can be disabled)
• in any case don't persist the object returned by python-grid5000 (leak issue)