It already adds it between prefix/name and randomly generated number.
As a nice side effect, this fixes test_dhcp_port_status_active test case
for plugins that don't support router names longer than 15 characters
(Cisco for Cisco Virtual Topology System is such a plugin).
If we need a test case that validates long names, there should be a
separate one that will validate it.
Change-Id: Ief08475787cb8fe2f13a78faac666fcad344f1f0