Justanshulsharma feature/encrypted data flag

@justanshulsharma Thanks again for the work you did on adding this feature! After some discussion we decided to modify the behavior somewhat, so I've added work to your PR here.

@serdar I don't have any more time to work on this before I leave. The code and unit tests for show and edit still need to be added. Also, I don't know if we have checks in place for what happens if knife.rb specifies `secret` and `secret_file` or if we need to add those to the code here.
I'm not a fan of having to provide a flag to this method.  But I need a way to differentiate between needing the `--encrypt` flag passed (for `data bag edit`) and not needing it (for `data bag show`).

I thought about having two different named public methods, that both call into a private method that takes the flag.  But I couldn't think of two good names to describe my API:

* 'Has the user provided a secret on the command-line or in the config file, with the --encrypt flag if the secret is in the file?'
* 'Has the user provided a secret on the command-line or in the config file?'

Those are the two APIs I am trying to provide.