Neutron DHCP implementation to raise exception if no ports have VIF

Currently only warning is risen, though deploy can't succeed in this
case for both PXE and Agent drivers.

Change-Id: If57a33818f5da820277c94b7b0ae642d29a698ce
Closes-Bug: #1362060
Merge "Neutron DHCP implementation to raise exception if no ports have VIF"