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)
NSX: allow net-migration only in combined mode

To allow migration of networks from the agent based model
to the agentless one, both modes need to be available; prior
to this change the extension was available in 'agentless' mode
too; this may lead to leftovers during the migration step.
This patch makes the extension available in 'combined' mode
only, which was the intended behavior.

Change-Id: I491ee3eaf25fc30c0ab4092ee198802a6b06d91d
Closes-bug: #1313985
(cherry picked from commit e737076c374d0743927782b6318132ecf226f05a)
Merge "NSX: allow net-migration only in combined mode" into stable/icehouse
Merge "NSX: ensure that no LSN is created on external networks" into stable/icehouse