Allows non-eureka based InstanceDiscovery.
Allows custom InstanceDiscovery.

fixes gh-566

fixes gh-417

Implements forwarding of eureka CacheRefreshedEvent as HeartbeatEvent,
rather thank serialization hack.  Uses new Eureka DI functionality
(mostly there, some issues). Updates EurekaJacksonCodec to be
"DataCenterAware". Reference EurekaClient interface rather than
DiscoveryClient class.

fixes gh-349

With the following configuration at `bootstrap.yaml`

```yaml

spring:
  application.name: spring-cloud-hystrix-dashboard
  profiles.active: prod
  cloud:
    config:
      discovery:
        enabled: true
        serviceId: spring-cloud-server-config-with-eureka
server.port: 9003

turbine:
  appConfig: some-service
  clusterNameExpression: 'default'
```

I wasn't able to see why Turbine can't find the instance. The reason was related to SPEL:

```
org.springframework.expression.spel.SpelEvaluationException: EL1008E:(pos 0): Property or field 'default' cannot be found on object of type 'com.netflix.appinfo.InstanceInfo' - maybe not public?
```

Anyways there was nothing there in the logs so maybe it should not be silenced like right now?

 fixes gh-147

Fixes gh-49