Third-party credentials are key-value pairs saved on the server.
They can be used from tasks (see tests for example).

Third-party credentials are stored encrypted in the database and are associated with a user.
Encryption relies on Scheduler's private key, so only the Scheduler can read credentials and
pass them to tasks.

Third-party credentials are passed along with existing CredData/Credentials objects since they
already existed and were exposed in tasks for runAsMe feature.

A specific key can be used: SSH_PRIVATE_KEY. It can be set as replacement for the SSH private
key embedded in credential files. This specific key takes precedence over a private key in the
credential file.

API calls for add/delete/list credentials have been added (Java & REST).

CLI commands to add/delete/list credentials have been added.

WITH THIS COMMIT, EXISTING CREDENTIALS NEED TO BE RE-GENERATED.

Change-Id: I6dedd67c722782d626b51683c0120af395b8b358