Add sections to the key-value store

[oleiade]

On the long run, I've often felt it would be easier to interact with trousseau if it supported sections: AKA thematic buckets in which key-value pairs would live.

For example having a "bitcoin" section containing everything about our beloved wallets, a "certificates" section and so on.

The syntax would be similar to kubernetes trousseau set mysection/mykey myvalue