backend: Store the BuxtonControl for direct access

In instances where direct access is used (CLI, daemon) the library
needs to gain access to the configuration used within BuxtonControl
in order to do layer operations, etc.

Instead of storing a BuxtonClient we'll store the BuxtonControl and
use that for direct access validation.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>