b2: Support Application Keys - fixes #2428

This supports B2 application keys limited to a bucket by making sure
we only list the buckets of the bucket ID that the key is limited to.

docs/content/b2.md

@@ -55,7 +55,7 @@ Choose a number from below, or type in your own value
 13 / Yandex Disk
    \ "yandex"
 Storage> 3
-Account ID
+Account ID or Application Key ID
 account> 123456789abc
 Application Key
 key> 0123456789abcdef0123456789abcdef0123456789
@@ -93,6 +93,21 @@ excess files in the bucket.
 
     rclone sync /home/local/directory remote:bucket
 
+### Application Keys ###
+
+B2 supports multiple [Application Keys for different access permission
+to B2 Buckets](https://www.backblaze.com/b2/docs/application_keys.html).
+
+You can use these with rclone too.
+
+Follow Backblaze's docs to create an Application Key with the required
+permission and add the `Application Key ID` as the `account` and the
+`Application Key` itself as the `key`.
+
+Note that you must put the Application Key ID as the `account` - you
+can't use the master Account ID.  If you try then B2 will return 401
+errors.
+
 ### --fast-list ###
 
 This remote supports `--fast-list` which allows you to use fewer