Vultr: Ensure self.returns is the source of truth (#44115)

As of today, self.returns it not the source of truth. If the return
value from querying the resource contains more values than the one
listed in self.returns, those value will be returned even though not
explicitly specified in self.returns.

This commit ensures that only the values listed on self.returns are
actually returned. The other values not listed are supressed.

lib/ansible/modules/cloud/vultr/vultr_startup_script.py

@@ -142,7 +142,7 @@ class AnsibleVultrStartupScript(Vultr):
         self.returns = {
             'SCRIPTID': dict(key='id'),
             'type': dict(key='script_type'),
-            'name': dict(key='name'),
+            'name': dict(),
             'script': dict(),
             'date_created': dict(),
             'date_modified': dict(),