Remove unused tenant_cidr from test_routers.py
Remove unused tenant_cidr from test_routers.py

tenant_cidr is not used in test_routers.py, this is to
remove it for code clean.

Change-Id: I2e7630a5a5a9ec718919b8cfd2ebe0b576c9a279

masayukig/tempest,masayukig/tempest,openstack/tempest,openstack/tempest,cisco-openstack/tempest,vedujoshi/tempest,Juniper/tempest,cisco-openstack/tempest,Juniper/tempest,vedujoshi/tempest