Fix a few bugs when using the fog deployer with a path specified

A few fixes for the fog deployer when using a path. In our particular use case we are deploying documentation for our different environments (staging, production) to separate prefixes in a single S3 bucket.

- Correctly compose the key for the file (path/to/file rather than pathto/file)
- Fix a bug where all files outside of the path get deleted when you deploy
- Fix a bug where fog throws a warning about "Unrecognized arguments: path"

I attempted to write tests for test_run_delete_stray when a path is set, but as the local fog provider doesn't have any concept of prefixes this wasn't possible. If you have any idea how this could be tested I'd be more than happy to implement it.