Fabric runner improvements and fixes

This branch includes the following changes and fixes:

1. Fix potential shell injection in the `rm` command (e.g., naming an action script file ";echo 'foo'" would result in a shell injection
2. Set executable bit when uploading script to the server. Previously we haven't done that, so if the file didn't have the executable bit set locally, the runner would explode.
3. Fix boolean parameter handling in the named_args. Previously, exception would be thrown if a boolean argument was provided as a named argument.