Bug 3893: provide well known url to know that net-virt topology is ready

Bug 3893: provide well known url to know that net-virt topology is ready

With this change, one can check the following url and know that ovsdb's netvirt
is up and functional:

   http://${ODL_IP}:8080/restconf/operational/network-topology:network-topology/topology/netvirt:1

Patch set 2: fix transaction
Patch set 3: code review, move new function to OvsdbInventoryServiceImpl
Patch set 4: typo (functional)

Change-Id: Ibe71d823b986fcdc9c12b6fa2a3994ae158fb696
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>