Rework HealthEndpoint and HealthIndicators

Refactored HealthEndpoint to be able to take multiple HealthIndicators. Extracted configuration of HealthIndicators out of EndpointAutoConfiguration and added new HealthIndicatorAutoConfiguration class.

Added HealthIndicators for Redis and Mongo.
Add unit test for (Mongo|Redis)HealthIndicator
Rename SimpleHealthIndicator to SimpleDataSourceHealthIndicator
Rename SimpleHealthIndicator to SimpleDataSourceHealthIndicator
Update documentation for new HealthIndicator support
Rename missing test
Merge pull request #819 from cdupuis/health-indicator

Rework HealthIndicator support