Add a "shell" exec driver.

This enables an administrator to run arbitrary commands as a docker
backend. This greatly simplifies debugging new backend drivers, as you
can quickly prototype exec drivers or potentially diagnose issues.

This also enables a simple "plugin" style system for exec drivers,
you simply need to provide a sensible wrapper script.

Docker-DCO-1.1-Signed-off-by: Adin Scannell <adin@scannell.ca> (github: amscanne)