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
aa7a4f297d