Floating IP show must reference router ID if exist

When a floating IP is associated to a port which its network is link to
a router, that floating IP should reference the router ID.
The schema change and logical router does not have anymore VMI ref for
interface. Only for the gateway.

Change-Id: Ic7d084fd745bbca37165b77f276019152a70740d
Closes-Bug: #1401827
Merge "Floating IP show must reference router ID if exist"
Return const refs to RouteDistinguisher and EthernetSegmentId

Automatic copy constructor and assignment operator do not work for
RouteDistinguisher and EthernetSegmentId.
Return const refs to these classes instead of returning by value.
The only exception is the static ::FromString method.

Change-Id: I1493d1ccf5dfeb8ef97b8d4c429c492cc61eddf5
Partial-Bug: 1399603