Added RBAC for REST APIs.

- admin role required for POST, PUT, DELETE & PATCH
- viewer role required for all other requests
- cleaned up all web.xml files for consistency and correctness

Change-Id: I33bad5cec0fb0f4285eed84173025b0a107b5aec
diff --git a/apps/segmentrouting/web/src/main/webapp/WEB-INF/web.xml b/apps/segmentrouting/web/src/main/webapp/WEB-INF/web.xml
index 73154e3..1362728 100644
--- a/apps/segmentrouting/web/src/main/webapp/WEB-INF/web.xml
+++ b/apps/segmentrouting/web/src/main/webapp/WEB-INF/web.xml
@@ -28,11 +28,13 @@
         </web-resource-collection>
         <auth-constraint>
             <role-name>admin</role-name>
+            <role-name>viewer</role-name>
         </auth-constraint>
     </security-constraint>
 
     <security-role>
         <role-name>admin</role-name>
+        <role-name>viewer</role-name>
     </security-role>
 
     <login-config>