NSX: fix validation logic on network gateway connect

NSX: fix validation logic on network gateway connect

This patch adds validation for the segmentation ID when the
network type for the gateway connection is vlan.
This will avoid requests with invalid vlan IDs are sent to
the backend resulting in 500 error responses being
returned to API users.

To this aim this patch slightly alters the current validation
logic due to the fact that some checks are unnecessary since
the same routine sets default values which avoid the
conditions being checked.

Change-Id: If0e71f6fdf27a49f0eda727e21405cffbc260a7a
Closes-Bug: #1340431