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
Merge "fix DeletableSubnet in api/network/common"