NSX: ensure that no LSN is created on external networks

NSX: ensure that no LSN is created on external networks

Logical Service Nodes (LSN) are those that provide dhcp
and metadata services to networks when the NSX plugin is
used without the openstack dhcp and metadata proxy
agents. External networks do not work with Neutron's
dhcp and metadata services, therefore there is no need
to provision a LSN in this case.

Added UT to avoid regression.

Closes-bug: #1316795

Change-Id: I3dab0c9cbecdd9296466c991d02d590d6bd5bd49
(cherry picked from commit af020da544ac4cf21b293d23710545ee5cdcdfa4)