Edges are left in TopologyManager database when nodes disconnect

When nodes are disconnected from ODL Controller, some edges are left
in TopologyManager's database. As a result GUI (and possibly REST APIs0
still shows some switches to be connected even when there are no
switches connected to the controller.

Change-Id: Ia4a5ae0160b07ba0207bf763dbc234e34dcf5816
Signed-off-by: Maurice Qureshi <maquresh@cisco.com>
Merge "Edges are left in TopologyManager database when nodes disconnect"