ECMP-fib entry not populated correctly

ECMP-fib entry not populated correctly

1. File: NeutronRouterChangeListener
- During Router Update, consolidating the original and updated routes,
the logics for calculation of the Routes to be added and routes to be
deleted were incorrect. It was deleting everything from the list
containing Routes to be deleted.

2. File: VpnInterfaceManager
- While deletion of adjacencies from odl-l3vpn:vpn-interface-op-data
datastore,  merge call was not removing it. Now using delete instead,
which is working.

3. File: VpnUtil
- Added InstanceIdentifier builder for reaching the correct adjacency
which would be deleted after merge.

Change-Id: I7299e642484c97dada3f966153b88d64c0cabf20
Signed-off-by: Amitesh Soni <amitesh.soni@ericsson.com>