Changed admission controller to allow volume expansion for all volume plugins

There are two motivations for this change:
(1) CSI plugins are soon going to support volume expansion. For such
plugins, admission controller doesn't know whether the plugins are
capabale of supporting volume expansion or not.
(2) Currently, admission controller rejects PVC updates for in-tree plugins
that don't support volume expansion (e.g., NFS, iSCSI). This change allows
external controllers to expand volumes similar to how external provisioners
operate.

pkg/controller/volume/events/event.go

@@ -30,4 +30,5 @@ const (
 	ProvisioningCleanupFailed = "ProvisioningCleanupFailed"
 	ProvisioningSucceeded     = "ProvisioningSucceeded"
 	WaitForFirstConsumer      = "WaitForFirstConsumer"
+	ExternalExpanding         = "ExternalExpanding"
 )