Required fields on resource rightscale_server instance should be optional

[douglaswth]

When specifying the instance portion of a rightscale_server resource, the image_href, instance_type_href, and name keys should not be required since they should either inherit from the ServerTemplate defaults in the case of image_href and instance_type_href or from the rightscale_server resource in the case of the name key since it is already required there.

resource "rightscale_server" "web_server" {
  name = "Web Server"
  ...
  instance {
    name = "Web Server"
    ...
  }
}