CLOUDSTACK-6047: Make sure every commands to VR resource is NetworkElementCommand
CLOUDSTACK-6047: Separate VR command generation and execution
CLOUDSTACK-6047: Generalize execution in VirtualRoutingResource
CLOUDSTACK-6047: Introduce GroupedAnswer

In some cases, Network Element need multiple answer in one, then introduced e.g.
IpAssocAnswer, SetFirewallAnswer, etc. But in fact they are basically the same.

So introduce GroupedAnswer for them.
CLOUDSTACK-6047: Add generic wrapper for group answer needed commands
CLOUDSTACK-6047: Introduce QueryCommand

QueryCommand is a kind of command which request the state rather than configure.
QueryCommands need to be executed immediately and cannot be aggregated.
CLOUDSTACK-6047: Introduce VR resource unit test