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)
NSX: do not raise on missing router during migration step

Return None for missing router rather raising NotFound exception;
this is what the client code was expected to handle in the first
place.

Closes-bug: 1313918

Change-Id: I573770a69d0170b027fc990c29aaa87b1fc8eccc
(cherry picked from commit 5e843850527ede7a56fcc24753f87c1e9e3e8bed)
Merge "NSX: fix error when creating VM ports on subnets without dhcp" into stable/icehouse