Add a guide to creating a credential of OpenStack


A guide to creating a credential of OpenStack is needed. Contributions are welcome 👍 

Related document: [A step by step guide to creating credentials of each cloud service provider](https://github.com/cloud-barista/cb-coffeehouse/blob/main/docs/A-step-by-step-guide-to-creating-credentials-of-each-cloud-service-provider.md)
