fix DeletableSubnet in api/network/common
fix DeletableSubnet in api/network/common

DeletableSubnet._router_ids was defined as a static attribute, causing
errors on tearDown working with multiple {router,subnet} couples as it
tries to disconnect subnet from all routers existing instead of all
routers connected to it

Change-Id: Id446d7bb2be117ea56b52e6a644b259089cdc158

Vaidyanath/tempest,redhat-cip/tempest,rakeshmi/tempest,Vaidyanath/tempest,CiscoSystems/tempest,alinbalutoiu/tempest,Lilywei123/tempest,queria/my-tempest,zsoltdudas/lis-tempest,masayukig/tempest,redhat-cip/tempest,ebagdasa/tempest,cloudbase/lis-tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,flyingfish007/tempest,cisco-openstack/tempest,yamt/tempest,yamt/tempest,jamielennox/tempest,flyingfish007/tempest,queria/my-tempest,neerja28/Tempest,pandeyop/tempest,sebrandon1/tempest,cloudbase/lis-tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,tudorvio/tempest,Lilywei123/tempest,manasi24/jiocloud-tempest-qatempest,BeenzSyed/tempest,hpcloud-mon/tempest,zsoltdudas/lis-tempest,akash1808/tempest,NexusIS/tempest,LIS/lis-tempest,JioCloud/tempest,rzarzynski/tempest,vmahuli/tempest,cisco-openstack/tempest,ebagdasa/tempest,tudorvio/tempest,Juraci/tempest,neerja28/Tempest,vedujoshi/tempest,roopali8/tempest,bigswitch/tempest,CiscoSystems/tempest,ntymtsiv/tempest,rzarzynski/tempest,afaheem88/tempest_neutron,afaheem88/tempest_neutron,pczerkas/tempest,pandeyop/tempest,ntymtsiv/tempest,afaheem88/tempest,hpcloud-mon/tempest,Juraci/tempest,Mirantis/tempest,rakeshmi/tempest,dkalashnik/tempest,roopali8/tempest,armando-migliaccio/tempest,jamielennox/tempest,xbezdick/tempest,alinbalutoiu/tempest,eggmaster/tempest,vedujoshi/os_tempest,nunogt/tempest,tonyli71/tempest,armando-migliaccio/tempest,Tesora/tesora-tempest,afaheem88/tempest,Mirantis/tempest,nunogt/tempest,jaspreetw/tempest,eggmaster/tempest,FujitsuEnablingSoftwareTechnologyGmbH/tempest,BeenzSyed/tempest,adkerr/tempest,varunarya10/tempest,manasi24/tempest,adkerr/tempest,JioCloud/tempest,Juniper/tempest,NexusIS/tempest,Tesora/tesora-tempest,vmahuli/tempest,hayderimran7/tempest,vedujoshi/tempest,LIS/lis-tempest,dkalashnik/tempest,tonyli71/tempest,bigswitch/tempest,xbezdick/tempest,izadorozhna/tempest,sebrandon1/tempest,varunarya10/tempest,openstack/tempest,jaspreetw/tempest,manasi24/jiocloud-tempest-qatempest,hayderimran7/tempest,akash1808/tempest,pczerkas/tempest,danielmellado/tempest,manasi24/tempest,izadorozhna/tempest,danielmellado/tempest,vedujoshi/os_tempest