create the executable path helper

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
new exception to indicate a missing executable

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add tests for the system utility

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add tests for the executable not found exception

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
use the new executable path helper in osd

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>