Support hdfs path as 'hdfs://nameservice'

[kyungwan-nam]

What would you like to be added:

we have to specify the bucket as namenode:rpcPort like below.

juicefs format \
    --storage hdfs \
    --bucket nn1.example.com:8020,nn2.example.com:8020 \
    --access-key hdfs \
    ... \
    myjfs

it looks good if we can specify the bucket as a URI format like hdfs://nameservice

juicefs format \
    --storage hdfs \
    --bucket hdfs://example \
    --access-key hdfs \
    ... \
    myjfs

nameservice can be converted as a comma-separated list of namenode:rpcPort by referencing hadoop conf

Why is this needed:

Normally, a hdfs path is represented as a URI format like hdfs://nameservice instead of namenode:rpcPort.
It will be intuitive and easy for users

[zhijian-pro]

done by #3577