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
Revert "Big Switch: Check source_address attribute exists"

This reverts commit 0dde14c0cd6ffea8ebff715342852ef17a9c0b70.

This fix only addressed part of the problem. The socket call
in python 2.6 does not accept an extra source address arg
either.

Change-Id: I4df4f66e9db0da1a8923ee37e918718f3bcf26ee
Partial-Bug: #1316382
Merge "NSX: ensure that no LSN is created on external networks"
Merge "Revert "Big Switch: Check source_address attribute exists""