Update upsun.json schema validator (#241)

* Update upsun.json

Trying to align the Upsun doc with the Yaml schema described here.
Related doc issue on specifics: 
* platformsh/platformsh-docs#4495
* platformsh/platformsh-docs#4497
* platformsh/platformsh-docs#4498
* platformsh/platformsh-docs#4499
* platformsh/platformsh-docs#4500
* platformsh/platformsh-docs#4501

Author: flovntp