CLOUDSTACK-5552 Setup private gateway state to ready on delete failure

On delete private gateway failure set the state to ready, So that it can be
deleted later.
CLOUDSTACK-5627: Removed a validation which expected createVPCOffering to fail if SourceNAT not provided

createVPCOffering API creates VPC with SourceNat even if not provided as param. So this validation
is not required any more.