Document untrusted certificates when connecting to hystrix.stream endpoint

This moves the lifecycle of the eureka client of of EurekaServiceRegistry.

fixes gh-2099

Otherwise useServlet31 was always returning false.

fixes gh-2196

Signed-off-by: João Salavessa <joao.salavessa@sky.uk>

Fixes gh-2125

Centralizing HTTP client creation and configuration

(Updated okhttp3 from v3.6.0 to v3.8.1)

They are all deprecated but that's no reason to break them.

Fixes gh-665

Fixes gh-1849