Add kerberos keytab authorization

[jo8937]

Thank you for your great work. embulk-output-hdfs is very cool plugin.
I want to add some feature - kerberos keytab auth.
use like this.

• command line auth

kinit -kt user.keytab user

• embulk yaml file

out:
  ....
  type: hdfs
  path_prefix: ./testdata
  file_ext: tsv
  config_files:
    - ./hadoop-conf/core-site.xml
    - ./hadoop-conf/hdfs-site.xml
  doas: user
  keytab_config:
    keytab_path: ./env/user.keytab
    keytab_principal: user@CLOUDERA.COM
    krb5_config_path: ./env/krb5.conf
  config:
    fs.defaultFS: 'webhdfs://my-hadoop-server:9870'
    fs.webhdfs.impl: 'org.apache.hadoop.hdfs.web.WebHdfsFileSystem'
    hadoop.security.authentication: 'kerberos'
    hadoop.security.authorization: 'true'
    dfs.client.use.datanode.hostname: 'true'
    dfs.namenode.kerberos.principal.pattern: 'hdfs/*@CLOUDERA.COM'    
   ....

This pull request's implementation is working fine in my environment with "webhdfs"

[jo8937]

Sorry for invalid PR.

[jo8937]

The keytab feature has some errors with hdfs scheme, but It works fine with webhdfs scheme
maybe it is useful for some user