Raise exception for network delete with subnets presents

N1kv plugin should raise an exception during network delete
if there is a subnet that is tied to that network.

Changed the order of the Assert arguments since now it's encoraged to have <expected_value, actual_value> now.

Change-Id: I92f177af0e2a2fbc460e0d668989ce590b7d1cb2
Closes-Bug: 1336107
Merge "Raise exception for network delete with subnets presents"