twister: Improve DUT release at DeviceHandler

Several improvements at Twister DeviceHandler when it releases
current DUT (Device Under Test):

 - release the exact DUT which is used for the test instance
   instead of all configured DUTs which happened to have
   the same serial device configured.

 - Twister PyTtest harness plugin adjustment to the above.

 - additional debug logging to track DUT waiting/retain/release.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>