Pin neutron and networking-generic-switch for ussuri
Pin neutron and networking-generic-switch for ussuri

neutron and networking-generic-switch has dropped support
of python2 with [1] and [2], let's pin these until RDO
moves to CentOS8. neutron patch has also caused issues
in promotion jobs as it is promoted to consistent repo because
unit tests are not running while building package [3].

[1] https://review.opendev.org/#/c/691874/
[2] https://review.opendev.org/#/c/697119/
[3] https://review.rdoproject.org/r/#/c/20991/

Change-Id: I72d585fca820c0ed6a7d45500cff59eee6f353ce

redhat-openstack/rdoinfo,javierpena/rdoinfo