NSX: ensure dhcp port is setup on metadata network

Change in scheduling behavior caused a regression
where the dhcp port is no longer provisioned on
the (admin) metadata network created when a subnet
is uplinked to a router.

This change recovers the past behavior and extend UT
coverage to avoid further regression.

Closes-bug: #1304127

Change-Id: I8420203f68a43368f3784adb0c4cbbe55f048662
(cherry picked from commit 4cb3eb2a98246f28f8016cfe32946d365203fbae)
Merge "NSX: ensure dhcp port is setup on metadata network" into stable/icehouse