Replace StagingInfo with StagingResult

The linux tailor now uses its own struct to create the staging_info.yml
file used by the DEA. The new StagingResult struct has an ExecutionMetadata
field instead of the more specific DetectedStartCommand. This change is for the
docker-circus.

[#78232658]

Signed-off-by: Max Brunsfeld <mbrunsfeld@pivotallabs.com>