[Tempest][Neutron] Add package name variable for OSP 15
[Tempest][Neutron] Add package name variable for OSP 15

In OSP 15 the neutron tests package starts with 'python3-'.

Change-Id: I5496d7ca3b28bccdbe5e1171ba2dafc37cd83115

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared