Allow override of discovery URL via ConfigMap

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

- Seems like the next step is to allow override of the CA bundle; I didn't
  do that here for simplicity of the commit, but seems like it is the right
  thing to do in the future.

pkg/config/api/types.go

@@ -7,7 +7,8 @@ package api
 
 // Config contains knobs to setup an instance of placeholder-name.
 type Config struct {
-	WebhookConfig WebhookConfigSpec `json:"webhook"`
+	WebhookConfig   WebhookConfigSpec   `json:"webhook"`
+	DiscoveryConfig DiscoveryConfigSpec `json:"discovery"`
 }
 
 // WebhookConfig contains configuration knobs specific to placeholder-name's use
@@ -21,3 +22,12 @@ type WebhookConfigSpec struct {
 	// to validate TLS connections to the WebhookURL.
 	CABundle []byte `json:"caBundle"`
 }
+
+// DiscoveryConfigSpec contains configuration knobs specific to
+// placeholder-name's publishing of discovery information. These values can be
+// viewed as overrides, i.e., if these are set, then placeholder-name will
+// publish these values in its discovery document instead of the ones it finds.
+type DiscoveryConfigSpec struct {
+	// URL contains the URL at which placeholder-name can be contacted.
+	URL *string `json:"url,omitempty"`
+}