Improves Test Kitchen's support for older (non-Linux) Unixes

Changes include:

* Convert all remote Bash shell code to Bourne (`/bin/sh`) compliant.
* Ensure Busser respects the `driver_config/sudo` config option.
* Update Busser to install all gems and plugins into a sandbox.
* Expose config options for: `BUSSER_ROOT`, Busser's Ruby binpath, and remote command `PATH`.
* Add a `proxy` driver for executing tests against long-lived, non-ephemeral test instances.

FUN FACT: These changes have enabled me to leverage Test Kitchen to execute a integration test suite against a physical network switch!

/cc @fnichol @lamont-granquist @sethvargo