Support kerberos keytab file

[kyungwan-nam]

What would you like to be added:

It is not possible to authenticate with a Kerberos keytab file when I use a kerberized hdfs as object storage.
JuiceFS support authenticating with a credential cache file only.
It would be good if support a keytab file as well.

Why is this needed:

Authenticating with a credential cache file is tricky under CSI Driver.
juicedata/juicefs-csi-driver#572

Currently, we have to do like the below whenever mount a pod. because the ccache file will be expired at the end.

• generate my ccache file by kinit
• create a kubernetes secret with my ccache file.

[kyungwan-nam]

I wrote the POC code, and It works well.
I can try to contribute if you are interested in this
thanks!

[tangyoupeng]

Thanks, we are interested in this.

[tangyoupeng]

@kyungwan-nam Hello, could you submit a pr for this issue. Some other users also encountered this problem.

[kyungwan-nam]

@tangyoupeng
Sorry for the delay.
Users can set the environment variables KRB5KEYTAB, KRB5PRINCIPAL for keytab instead of KRB5CCNAME.

[zhijian-pro]

done by #3517