Use system properties on the HttpClient that's used in J4pClientBuilder

At the moment, i can't find a good way to tell the jolokia client to use an http/https proxy. Using the http_proxy env variables don't work; maybe we can enhance the J4pClientBuilder to call HttpClient.custom().useSystemProperties() as part of it's call so we can set system props to configure the http proxy

related: https://github.com/fabric8io/fabric8/issues/2972
