BUG 6489: Routes to InterVpnLink not advertised on BGP

BUG 6489: Routes to InterVpnLink not advertised on BGP

 + Missing functionality: when a route pointing to an interVpnLink
   endpoint is added/removed to a Neutron Router, CSC must advertise
   the prefix. The nexthop(s) for the BGP are the DPNs where the
   endpoint of the InterVpnLink was instantiated

 + Moves some InterVpnLink utility methods from VpnUtil.java to
   InterVpnLinkUtil.java

 + Changes some return values to Optional.

Change-Id: I9e671b34400151099e501d26a8a7839c7730d0b7
Signed-off-by: Suraj Ranjan <suraj.ranjan@ericsson.com>