fix: improve command format spec.
fix: improve command format spec.

I am implementing commands data-schemas now.
https://github.com/screwdriver-cd/data-schema/pull/195

I think we should add `command` optional key  to the Docker format that allows users not to set an extra executable (knife, ...).
And we should rename `binary` key of the Habitat format to `command`.
Please review it.

screwdriver-cd/api