VTN Manager forgets flow entries when the controller enters container mode.

How to reproduce:

1. Create VTN and vBridge in the default container, and map physical network
to the vBridge.
2. Install flow entries by sending packets to the vBridge.
3. Create a container. When the first container is created, FRM uninstalls all
flow entries from switches, and VTN Manager forgets those flow entries.
4. Destroy the container that was created by step 3. When the last container
is destroyed, FRM restores all flow entries that were uninstalled by step 3.
But those flow entries are out of control because VTN Manager no longer
manages.

VTN Manager should:

• preserve flow entries during container mode.
• ignore FLOW_REMOVED notifications during container mode.
• behave as if no data flow is installed.