update requirements for neutron-lib 1.18.0

Neutron-lib 1.18.0 is our Rocky RC and is already being used by neutron
[1]. This patch updates the neutron-lib required version to match
neutron [1] in prep for the Rocky release.

[1] https://review.openstack.org/#/c/583671/

Change-Id: Iee7a1ab3559b3ef58354f90774c2854e41dc18ae
1 file changed