Support setting the path for hdfs

[kyungwan-nam]

What would you like to be added:

juices store data on the subdirectory of the root only (e.g. /myjfs) when setting hdfs as object storage.
https://juicefs.com/docs/community/how_to_setup_object_storage#hdfs
It looks good if we can set the path for object storage like --bucket namenode1:8020/user/kyungwan-nam

juicefs format \
    --storage hdfs \
    --bucket namenode1:8020/user/kyungwan-nam \
    --access-key hdfs \
    ... \
    myjfs

Why is this needed:

I have a multitenant HDFS cluster, and a lot of users share it.
each user has a home directory (e.g. /user/a/, /user/b/) separately. and the other path is not allowed.
thus, I need to change it to a path with permission, not the subdirectory of the root.