DIB_PYTHON_EXEC was added in I5fab0e192c3a2dad8f60e821c184479e24e33bcd
to export the python that disk-image-create is running under. If dib
is installed with python3 then just calls "python" (python2) to run
sub-scripts, it fails to find itself.
This has been tested in devstack gates that use py3x
Change-Id: Ia1028972bfc0517b468b279aab9decdbcd7424ca