Use ? in names of predicate. Delete unused method.

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Use idiomatic as_json interface for diego requests

[#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>
Get rid of DesireAppMessage

- we do not need the validation it provides and do not do this for staging requests
- also convert Diego::Environment to using the as_json idiom along the way
- [#75217152]

Signed-off-by: Peter Parente <pparent@us.ibm.com>