Add a cloud-provider hook to scrub DNS for pods

GCE needs this hook and it seems general enough to include.

pkg/cloudprovider/cloud.go

@@ -39,6 +39,8 @@ type Interface interface {
 	Routes() (Routes, bool)
 	// ProviderName returns the cloud provider ID.
 	ProviderName() string
+	// ScrubDNS provides an opportunity for cloud-provider-specific code to process DNS settings for pods.
+	ScrubDNS(nameservers, searches []string) (nsOut, srchOut []string)
 }
 
 // Clusters is an abstract, pluggable interface for clusters of containers.