uniquely identifying configs by external id

[adityathebe]

fix source vertex not found error by namespacing all look ups to the scraper.

The source vertex not found error would occur only in cases where multiple clusters were being scraped. The lookups needed to be scoped to the given scraper.

Example: When forming a Namespace default -> Deployment Nginx relationship for cluster A, the lookup for external id Kubernetes/Namespace//default would return the ID of the default namespace from cluster B. This would cause failure when forming the graph as the edge couldn't be inserted since there's no source vertex in the graph.

resolves: #703