vm.Instance to use generic data field for extra info

The Instance struct currently looks like this:

type Instance struct {
	Name string

	IPAddress string

	// For machine authentication
	Username      string
	Password      string
	PrivateSSHKey []byte

	GCP gcp.Instance
}

Where we have an explicit gcp.Instance field, and the vm package needs to import a autoscaler/gcp/config (alised to gcp).

To solve this leaky abstraction, I propose we instead use:

type Instance struct {
	Name string

	IPAddress string

	// For machine authentication
	Username      string
	Password      string
	PrivateSSHKey []byte

	// Data is underlying provider-specific instance data
	Data interface{}
}

The only place Data will be used is in a provider implementation, which is fully aware of what type will be stored within this field, and can easily vmInst.Data.(config.Instance) to cast to the type it knows should be present.

Our use of interface{} should correctly be rare, but I think this is a specific instance where we should allow it. It reminds me of similar situations you'll find in the Go runtime, such as with FileInfo (https://golang.org/pkg/io/fs/#FileInfo) and being able to fetch the underlying data regarding a dir entry with Sys() interface{}.