Add docstring for tenant_network
Add docstring for tenant_network

create_test_server() method supports tenant_network, but its
docstring doesn't contain it. So this patch adds it.

Change-Id: Idb7786bb43ff79b4503e4f9b779c8ee964c313db

Juniper/tempest,sebrandon1/tempest,flyingfish007/tempest,masayukig/tempest,vedujoshi/tempest,LIS/lis-tempest,bigswitch/tempest,vedujoshi/tempest,tonyli71/tempest,dkalashnik/tempest,zsoltdudas/lis-tempest,Juniper/tempest,zsoltdudas/lis-tempest,Tesora/tesora-tempest,flyingfish007/tempest,cisco-openstack/tempest,rakeshmi/tempest,izadorozhna/tempest,pczerkas/tempest,rakeshmi/tempest,tonyli71/tempest,bigswitch/tempest,openstack/tempest,Tesora/tesora-tempest,pczerkas/tempest,dkalashnik/tempest,openstack/tempest,masayukig/tempest,cisco-openstack/tempest,xbezdick/tempest,xbezdick/tempest,sebrandon1/tempest,LIS/lis-tempest,izadorozhna/tempest