Use /usr/bin/env, not /bin/env
Use /usr/bin/env, not /bin/env

/usr/bin/env is more portable (/bin/env does not exist on SUSE).
Also address minor nit on review about python 3.x compatibility.

Change-Id: I6bc1a579bb43664f4d2be35a96f45189a6df0e27

openstack/diskimage-builder,eantyshev/diskimage-builder,varunarya10/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,NaohiroTamura/diskimage-builder,JioCloud/diskimage-builder,citrix-openstack-build/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,NaohiroTamura/diskimage-builder,citrix-openstack-build/diskimage-builder,eantyshev/diskimage-builder,rdo-management/diskimage-builder,JioCloud/diskimage-builder,switch-ch/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,rdo-management/diskimage-builder,varunarya10/diskimage-builder,openstack/diskimage-builder