Use workflow id in Mistral REST API

currently, we identify a workflow by its name, of cource, the workflow name is unique within a tenant. However, when we use 'public' workflows or we want to get benifits from workflow sharing feature[1], we may see more than one workflows with the same name (you can see a related bug here[2]). Even worse, users will never see other 'same-name' workflows when performing 'mistral workflow-get <name>' command, since it always returns the first one.

Look at almost all other projects, they always use UUID as globally unique resource identifier, especially in the REST API, the resource name is a string that can be duplicated throughout the whole system, and may be changed frequently as the time goes by.

So, I propose we use id as the workflow identifier in the REST API. What's more, we can consider using id for other resources after that.

[1]: https://blueprints.launchpad.net/mistral/+spec/mistral-resource-sharing
[2]: https://bugs.launchpad.net/python-mistralclient/+bug/1518276