[Feature Request] Allow bucket name to be specified in Backblaze B2 backend setup for single bucket keys.

[hector-hyrivera]

If you create an application key that only has access to a single bucket, it will fail in the Init. This is due to B2 declining all "list_buckets" request if the key is limited to one bucket and the bucket name is not in the request. (See https://b2-sdk-python.readthedocs.io/en/master/api/api.html#b2sdk.v2.B2Api.list_buckets)

I believe it would be beneficial to add the ability to pass in the bucket name so we can limit which buckets are exposed to the application key we generate for Filestash.

[mickael-kerjean]

That's what the path is about, just put: /bucketname/ and you will be chrooted onto that bucket