For anyone wanting to use this script with SNAP 11, you would have to change:
import snappy as snap
to
import esa_snappy as snap
and
create_stack = snap.jpy.get_type('org.esa.s1tbx.insar.gpf.coregistration.CreateStackOp')
to
create_stack = snap.jpy.get_type('eu.esa.sar.insar.gpf.coregistration.CreateStackOp')