Address bug that may generate two --rsync-path options with Rsync, #1356

[sdondley]

This PR fixes #1356 by prefixing any existing value of --rsync-path with "sudo" if sudo keyword doesn't already exist in the value.

Checklist

• [X] based on top of latest source code
• [ ] changelog entry included
• [x] tests pass on Travis CI
• [x] git history is clean
• [x] git commit messages are well-written