Document global scoped rewrite rule option 'gateway.global.rules.services'

As part of KNOX-711 rewrite rules were made scoped instead of global i.e. rewrite rules defined for service will be the only ones used (rewrite rules from other services will not be used).

For backwards compatibility some services needed global rules, as a result an option was added to make rewrite rules defined in a service global

<property>
        <name>gateway.global.rules.services</name>
        <value>"NAMENODE","JOBTRACKER", "WEBHDFS", "WEBHCAT", "OOZIE", "WEBHBASE", "HIVE", "RESOURCEMANAGER"</value>
    </property>