Configurable front end URL for simplified load balancer configuration

Given the amount of URL rewriting that Knox does both in headers and in response content it may not be practical to configure load balancers to do the same level or rewriting to redirect the use of those URLs back through the load balancer. Therefore it should be possible to configure Knox with the URL that should be used during rewriting so that it can rewrite the URLs with the correct "frontend" URL so that the load balancer does not have to do any rewriting.

This patch adds a configuration setting to gateway-site.xml

    <property>
        <name>gateway.frontend.url</name>
        <value>http://localhost/gateway</value>
    </property>