Operator switching CC Diego flags correctly switches behaviour of apps

Also, refactor bulk endpoints to use backends
Simplify Backends interface, validate StagingCompletion only handles Diego apps.
Refactor Backends into three

* Stagers - know about staging
* Runners - know about running
* InstancesReporters - know how to report

HM9000 needs to check for Diego on stop app

* When Diego is enabled, the HM9000 needs to stop any app that is marked
* for running on Diego.
[#80165482]

Signed-off-by: Nick Wade <nwade@pivotal.io>