State upgrade step: Add EnvUUID to cleanup docs.

Add an upgrade step which stringifies the cleanup doc (bson.ObjectId)
and prepends State's environment UUID. Update state.docID() to take an
interface and return a string.
fix conflicts
Merge pull request #994 from waigani/migrate-cleanup

State upgrade step: Add EnvUUID to cleanup docs.

Add an upgrade step which stringifies the cleanup doc (bson.ObjectId)
and prepends State's environment UUID. Update state.docID() to take an
interface and return a string.