Implement infrastruture discovery agent core

[mohabusama]

Right now we have multiple infrastructure discovery agents used to sync different third-party system components to ZMON entities (e.g. AWS, Kubernetes, GCP ...). Each agent is standalone, however a lot of code is identical/repeated.

The idea is to implement a core agent which can dynamically load plugin(s) to discover different external system components, and the core syncs them with ZMON backend. That way we enable easier integration points with external systems.

Useful here as well: zalando-zmon/zmon-demo#10