Dan/sneha using helper methods to extract metadata_entries

introduced MetadataHelper, which converts a complex metadata entry hash
into metadata key_value pair.
the idea here is to use this helper by different entities like vm, vapp
to represent metadata in same way
Also added specs to cover different types of metadatas.
cleaning fog interface. moving metadata code to a mixin
including compute_metadata behavior in compute classes
adapting test to fetch metadata using core
added missing new line
removed duplicate spec
extracted out metadata value types
correctly renaming a variable
Merge pull request #69 from alphagov/walker_should_report_metadata

core blocks now provide compute metadata