NSX: fix error when creating VM ports on subnets without dhcp

The error was caused by a missing LSN port (which is created when
the subnet is setup with dhcp enabled). Therefore, check the port
is in place before proceeding with the setup.

Change-Id: I658ead0c324a24ad3f061844ecac13cd3359d341
Closes-bug: 1316365
(cherry picked from commit 7458427a0018437f4c7cac127bcd03c93cccfe70)
Merge "NSX: fix error when creating VM ports on subnets without dhcp" into stable/icehouse