MXS-3013: Add reload endpoint for services

The endpoint reloads the database users. This can also be used in the
future to refresh encrypted credentials when they are no longer loaded
every time an encrypted password is used.

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

@@ -168,5 +168,10 @@ describe("Service", function() {
             .should.be.fulfilled
     });
 
+    it("reload users", function() {
+        return request.post(base_url + "/services/RW-Split-Router/reload")
+            .should.be.fulfilled
+    });
+
     after(stopMaxScale)
 });