core blocks now provide compute metadata

This refactoring was required as the part of story - walker should report vapp & vm metadata.

The logic to fetch metadata was located in ```fogServiceInterface```, we have moved it to achieve following:

- ```fogServiceInterface``` should simply query metadata and should not have logic to extract name/value pairs from metadataEntry hash. This logic is now moved to ```MetadataHelper```. 

- introduced a mixin ```ComputeMetadata```, which provides metadata behaviour for compute entities like Vapp and Vm. This is because metadata works similarly for compute entities. 


@danielabel & Sneha pairing