Fail to list VPCs due to invalid json schema

[Boris V.Kuznetsov]

Environment:

ACS: 4.16.0
OS: Ubuntu 21.10
SDK: v.2.12.0

Issue:
SDK fails to unmarshall listVPCs result when there are network tiers attached to a VPC. Here's my output:
VPCService_test.go:69: Failed to list VPCs due to: json: cannot unmarshal object into Go struct field VPC.vpc.network of type string

[David Jumani]

Fixed in #26