Get rid of stdin for deis config:push

The implementation of `deis config:push` [here](https://github.com/deis/workflow-cli/blob/master/cmd/config.go#L238) uses `stdin` to fetch the buffer, but it was causing me problems as you can see at this [issue](https://github.com/concourse/concourse/issues/1192). Running the command inside a script which gets its inputs from stdin, I had to pipe it with `< dev/null` to get the correct result or it would give me `Creating config... ...Error: Unknown Error (409): {"detail":"user changed nothing - release stopped"}`.

Can something be made to correct it?