lint fixes for volume/*

- comments on exported values
 - constant string replaced by constant reference
 - unexport implementation details of VolumeDriver 'local'
 - add fixed packages to linter list

Signed-off-by: Morgan Bauer <mbauer@us.ibm.com>

hack/make/validate-lint

@@ -63,6 +63,9 @@ packages=(
 	registry
 	runconfig
 	utils
+	volume
+	volume/local
+	volume/drivers
 )
 
 errors=()