Reject any docker_image and buildpack combo at staging time

[#75859568]
Add a DockerPolicy to constrain docker use

Initially, prevent docker_image and buildpack from being set together

[#75859568]
Add diego_docker flag to ymls

[#75859500]

Signed-off-by: Stewart Nickolas (nickolas@us.ibm.com)
Prevent push of docker_image when diego and/or diego_docker are disabled

Add test cases for validation errors and ensure flags are set appropriately so that existing specs continue to test as intended.

[#75859500]
Var and method name improvements after code review

[#75859500]