Remove 'QDROUTERD' from the image override environment variables

[ted-ross]

I propose that we change QDROUTERD_[IMAGE|IMAGE_PULL_POLICY] to use the text SKUPPER_ROUTER instead.

const (
	RouterImageEnvKey                 string = "QDROUTERD_IMAGE"
	ServiceControllerImageEnvKey      string = "SKUPPER_SERVICE_CONTROLLER_IMAGE"
	ConfigSyncImageEnvKey             string = "SKUPPER_CONFIG_SYNC_IMAGE"
	RouterPullPolicyEnvKey            string = "QDROUTERD_IMAGE_PULL_POLICY"
	ServiceControllerPullPolicyEnvKey string = "SKUPPER_SERVICE_CONTROLLER_IMAGE_PULL_POLICY"
	ConfigSyncPullPolicyEnvKey        string = "SKUPPER_CONFIG_SYNC_IMAGE_PULL_POLICY"
	SkupperImageRegistryEnvKey        string = "SKUPPER_IMAGE_REGISTRY"
)