[Neutron] Fix undef hiera value for gre protocol

We have to use 'None 'in python to pass 'undef' to hiera.

Change-Id: I72fc58c765b2fc67a2b27c677fd1fa1e9762244f
(cherry picked from commit e95098f5d5f21f830664608eebf964d4c46e3796)