MXS-1220: Add /users/ resource

The /users/ resource shows all user accounts that can be used with
MaxScale. This resource is further split into two resource collections,
/users/inet and /users/unix, which contain the network users and Linux
accounts respectively.

server/core/test/rest-api/test/schema_validation.js

@@ -14,6 +14,9 @@ describe("Resource Collections", function() {
         "/maxscale/threads",
         "/maxscale/modules",
         "/maxscale/tasks",
+        "/users",
+        "/users/inet",
+        "/users/unix",
     ]
 
     tests.forEach(function(endpoint) {