Add support for a separate secrets.yml file stored on the file system

In our company we have a production jenkins master next to one in acceptance and test. All our changes to jenkins configuration are first tested on test before staging to accept and production. Our config contains a lot of environment specific configuration (e.g. Jira URL, proxy configuration, ...). To facilitate the use of one configuration file with variables for those environment specific items I suggest to add a new secretsource that reads a file (e.g. secrets.yml) from the file system to reveal the variables. This way the environment config can be separated from the master file.