Use the dib python to do cleanup
Use the dib python to do cleanup

In a system where python2 is not installed and /usr/bin/python is not
linked then the cleanup process will fail trying to invoke the python
script. Use the previously determined DIB_PYTHON_EXEC if it's available.

Change-Id: I128292808ccef92cc1803988b35caae5aa6fa541

switch-ch/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder