Support MapReduce executor

[sakama]

Embulk supports distributed executor such as an embulk-executor-mapreduce.

This executor run tasks on remote server and couldn't read local file path that is written at
 config.yml.
Because file is not there.

I changed secret_key_file parameter to accept embedded secret key.

out:
  type: sftp
  ...
  secret_key_file:
    content |
      -----BEGIN RSA PRIVATE KEY-----
      ABCDEFG...
      HIJKLMN...
      OPQRSTU...
      -----END RSA PRIVATE KEY-----
  ...

This changes keeps a backward compatibility.
So we can set local file path as before.

out:
  type: sftp
  ...
  secret_key_file: /path/to/id_rsa
  ...

[sakama]

CI failed. This failure will be fixed by #7.

[sakama]

#7 was merged. And I merged these changes.

I guess building environment for MR-Executor is little hard.
I welcome that you assign issue to me when something wrong behavior is reported.

[civitaspo]

np!

[sakama]

Thank you!