Add the ability for ribbon clients to specify response status codes they would like to retry

Document Hystrix timeout configuration with Ribbon clients

* feature/exceptions-monitoring:
  Implements zuul CounterFactory

CounterFactory is used by ZuulException and was previously initialzed
with a dummy impl.

This adds a Spring Boot CounterService based implementation.

Also adds the ability to implement a zuul TracerFactory.

* brainlessly apply this : https://github.com/Netflix/Hystrix/commit/892f6a781e67f153b0bf2f7387c87ca0721955c2

Add the ability for ribbon clients to specify response status codes they would like to retry.  Fixes #1540.

fixes gh-1824

Use valid hostnames in feign tests

Change /routes endpoint to use EndpointMvcAdapter and AbstractEndpoint.  

Fixes #1797

Fixes the addition of http with EL syntax

For more advanced filter examples.

fixes gh-1748

Make EurekaDiscoveryClient.EurekaServiceInstance constructor public

fixes gh-1792

fixes gh-1786

This reverts commit a628ba1b.

GH-1334: Support for eagerly initializing zuul Ribbon contexts

By default, `LoadBalancerBuilder` built a load-balancer with a `PollingServerListUpdater`. This change introduces a `ServerListUpdater` bean which creates a `PollingServerListUpdater(IClientConfig)` so that `ribbon.ServerListRefreshInterval` is respected.

 fixes gh-1304

Update docs as @Autowired fails with many beans

Fix a suspected typo. @Autowired throws a `NoUniqueBeanDefinitionException` if there are multiple beans of the same type without a primary, but the documentation said multiple beans 'cause `@Autowired` to work', rather than 'not work'.

fixes gh-1766

 Previously, when feign used the Ribbon RestClient, an exception was thrown if Content-Length was set. Over time, feign has moved away from RestClient, yet removing the header still happened.

 Fixes gh-1705