Refactor pkg/util/mount to be more reusable

This patch refactors pkg/util/mount to be more usable outside of
Kubernetes. This is done by refactoring mount.Interface to only contain
methods that are not K8s specific. Methods that are not relevant to
basic mount activities but still have OS-specific implementations are
now found in a mount.HostUtils interface.

pkg/volume/plugins.go

@@ -334,6 +334,8 @@ type KubeletVolumeHost interface {
 	CSIDriversSynced() cache.InformerSynced
 	// WaitForCacheSync is a helper function that waits for cache sync for CSIDriverLister
 	WaitForCacheSync() error
+	// Returns HostUtils Interface
+	GetHostUtil() mount.HostUtils
 }
 
 // AttachDetachVolumeHost is a AttachDetach Controller specific interface that plugins can use