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
Merge "NSX: ensure that no LSN is created on external networks"