title: CloudFormation SecretTargetAttachment return SecretString is not valid JSON
body: <p>I am trying to create a simple CloudFormation Stack, but it does not work. Here is my CloudFormation template.</p>
  
  <pre><code>Resources:
    MyDBSecrets:
      Type: AWS::SecretsManager::Secret
      Properties: 
        Description: 'This is password of mysql database'
        GenerateSecretString:
          PasswordLength: 16
          ExcludePunctuation: true
        Name: MyDBSecrets
    MyDBInstance:
      Type: AWS::RDS::DBInstance
      Properties:
        DBName: MyDBInstance
        AllocatedStorage: '20'
        DBInstanceClass: db.t3.micro
        Engine: mysql
        MasterUsername: 'testdb'
        MasterUserPassword: !Join ['', ['{{resolve:secretsmanager:', !Ref MyDBSecrets, ':SecretString}}' ]]
    SecretRDSInstanceAttachment:
      Type: "AWS::SecretsManager::SecretTargetAttachment"
      Properties:
        SecretId: !Ref MyDBSecrets
        TargetId: !Ref MyDBInstance
        TargetType: AWS::RDS::DBInstance
  </code></pre>
  
  <p>On stack creation I can see my secrect resource is created, my RDS instance is also created, but on  SecretTargetAttachment I am getting <code>CREATE_FAILED with 'SecretString is not valid JSON'</code> error. Am I missing something ?</p>