Add database relationship between router and ports

Add an explicit schema relationship between a router and its ports. This
change ensures referential integrity among the entities and prevents orphaned
ports.

Change-Id: I09e8a694cdff7f64a642a39b45cbd12422132806
Closes-Bug: #1378866
(cherry picked from commit 93012915a3445a8ac8a0b30b702df30febbbb728)