Fix network profile subtype validation in N1kv plugin

Network profile of type Trunk expects a subtype field for
creation. If a subtype is missing in the request body, plugin
should raise an exception. This patch fixes the validation logic
to detect missing subtype field.

Change-Id: I6b4fb533576a317f7d568b4a0fe4117b5464ffa6
Closes-Bug: 1315197
(cherry picked from commit 1dd0bdc06dedf0f577d220f4c1a1f9f43f04a0fe)
Merge "Fix network profile subtype validation in N1kv plugin" into stable/icehouse