storage metric: add generic implementation

Add the generic implementation for the storage metric, so there is no
need to add a new enum value and new code, every time a new storage
type needs to be added.

Change-Id: Idcd99455ca559eb6274a71613684a99b4b63a6f0
Signed-off-by: Jagpal Singh Gill <paligill@gmail.com>
diff --git a/health_metric_config.hpp b/health_metric_config.hpp
index 6391462..dedf2f4 100644
--- a/health_metric_config.hpp
+++ b/health_metric_config.hpp
@@ -36,9 +36,7 @@
     memoryFree,
     memoryShared,
     memoryTotal,
-    // Storage subtypes
-    storageReadWrite,
-    storageTmp,
+    // Types for which subtype is not applicable
     NA
 };