Improve code readability
Fix the metadata dictionary checks:
 1- Raise error correctly and separeta the "None" check from the malformed check
 2- Provide comments explaining the odd GCE dictionary format
 3- Perform check on metadata dictionary structure. We expect one "items" key
    and a tuple of arbitrary values.
 4- Prefix the key/value provided by a simple dictionary by "items", if not found,
    enforcing point 3's structure.