Guard Master/RS Dump Servlet behind admin walls

Currently RSDumpServlet and MasterDumpServlet do not require any check for whether the user has privileges to access to instrumentation servlets.

This is unlike other servlets like ProfileServlet, ConfServlet, JMXJsonServlet etc. which are guarded by admin checks.

Goal of this JIRA is to add similar check for RS and Master Dump Servlet. Post this change only admins will be able to access RSDumpServlet and MasterDumpServlet, if hadoop.security.instrumentation.requires.admin is enabled.