make setup/turnoff of tap devices optional

[wjhun]

In ops/cmd/run_local_instance.go, network.SetupNetworkInterfaces() and network.TurnOffNetworkInterfaces() are called unconditionally if a tap device is specified. This is convenient behavior when the user does not already have bridge and tap devices configured, but an inconvenience if the user has such devices configured and wishes them to be persistent across invocations of ops. Adding a flag to disable all network device configuration (or disable it by default) would help in these cases.