feat (engine): Multiple Instance now works with empty collections

closes CAM-879
improve(cockpit): no such engine error message

- improve "no such process engine" error message
- fix select box markup bug

related to #CAM-879
fix(cockpit.multi-tenacy): use Cockpit service to resolve default engine

- engine is now provided soley via ProcessEngineProvider SPI
- consolidates the cockpit+rest api structure

Related to CAM-879
Merge branch 'master' of github.com:camunda/camunda-bpm-platform