ISSUE-218 Makes converter replacement char configurable thru config.

This PR is for issue https://github.com/Netflix/eureka/issues/218
The main idea to make char replacement configarable thru EurekaClientConfig. The default stay as is.
Can you please remove the static here? The ClientConfig instance is a singleton so should function fine as an instance variable. The *XStream classes are also used on the server side and we have seen cases where statics can cause bootstrapping issues (#214).