VCH Creation API: Decide what to do if RoutingDestinations is empty

[zjs]

Currently, the fromGateway method used by the VCH creation API handlers is unclear about the behavior if the supplied RoutingDestinations are empty. We should determine what work, if any, is required here.

[andrewtchin]

determine when routingdestinations is required (should match cli) and figure out if we can use swagger features for validation

[andrewtchin]

Related #6400

[zjs]

Relevant lines from the vic-machine-* CLI help output:

   --client-network-gateway value                   Gateway for the VCH on the client network, including one or more routing destinations in a comma separated list, e.g. 10.1.0.0/16,10.2.0.0/16:10.0.0.1
   --public-network-gateway value                   Gateway for the VCH on the public network, e.g. 10.0.0.1
   --management-network-gateway value               Gateway for the VCH on the management network, including one or more routing destinations in a comma separated list, e.g. 10.1.0.0/16,10.2.0.0/16:10.0.0.1
   --container-network-gateway value, --cng value   Gateway for the container network's subnet in CONTAINER-NETWORK:SUBNET format, e.g. vsphere-net:172.16.0.1/16

Based on this, it would appear that the CLI's requirements differ slightly depending on the type of network:

• Client: one or more
• Public: zero
• Management: one or more
• Container: exactly one