TEP not deleted when subnet is deleted

We had some downstream scaling/performance testing with browbeat/rally, and after the neutron scenarios are executed, we see ODL resources not being cleaned up, including TEP.

Steps to recreate:

1. Bring up devstack (stable/newton) and ODL (master... also broken in stable/carbon).

2. Show TEPs
opendaylight-user@root>tep:show
Tunnel Monitoring (for VXLAN tunnels): On
Tunnel Monitoring Interval (for VXLAN tunnels): 1000

TransportZone TunnelType SubnetMask GatewayIP VlanID DpnID IPAddress PortName
------------------------------------------------------------------------------------------------------------------------------
9fff92ad-09b0-4db1-a2c8-0b9b94bbb2f2 VXLAN 0.0.0.0/0 0.0.0.0 0 83568199077816 10.8.125.240 tunnel_port
777f2002-93c8-42c6-9e15-8e4e055bed9c VXLAN 0.0.0.0/0 0.0.0.0 0 83568199077816 10.8.125.240 tunnel_port

3. Create network
neutron subnet-create vx-net 10.100.5.0/24 --name vx-subnet

4. Create subnet
neutron subnet-delete vx-net

3. Display TEPs
opendaylight-user@root>tep:show
Tunnel Monitoring (for VXLAN tunnels): On
Tunnel Monitoring Interval (for VXLAN tunnels): 1000

TransportZone TunnelType SubnetMask GatewayIP VlanID DpnID IPAddress PortName
------------------------------------------------------------------------------------------------------------------------------
9fff92ad-09b0-4db1-a2c8-0b9b94bbb2f2 VXLAN 0.0.0.0/0 0.0.0.0 0 83568199077816 10.8.125.240 tunnel_port
777f2002-93c8-42c6-9e15-8e4e055bed9c VXLAN 0.0.0.0/0 0.0.0.0 0 83568199077816 10.8.125.240 tunnel_port
94899c9b-bf55-48dd-a22e-1d9bf9e36f3b VXLAN 0.0.0.0/0 0.0.0.0 0 83568199077816 10.8.125.240 tunnel_port

Notice there is a new, 3rd TEP from subnet creation.

4. Delete subnet
neutron subnet-delete vx-net

5. Show TEP, 3rd TEP is still there!