parameter value base64 encoding

Add the ability to specify that the value of a parameter should be base64
encoded before it gets sent to the cloud orchestration environment.

If you have a yaml parameters file, you can do the following

myParameter:
  base64: |
    This will be encoded.

When thunder encounders a value that is a hash contining the key "base64", it will
substitute the base64 encodeing of the value. An equivalent parameters file would be

myParameters: VGhpcyB3aWxsIGJlIGVuY29kZWQuCg==