Workflow sharing support in Mistral

The idea is from https://bugs.launchpad.net/mistral/+bug/1470373, although we already have a solution for that, I think we should think more, to make the feature more flexible, more controllable to end users. Think about scenarios below:

* When user A creates workflow/workbook/action via API, he can specify a param (e.g. 'scope=public' ) , then all the mistral users can see and use that resource user A created.
Yeah, it's already resolved by bug 1470373! But there is a potential problem, what if user B doesn't want to see so many 'noisy' workflows/actions?

* User A created a 'private' resource as before, he makes a POST request to mistral, e.g. http://localhost:8989/workflows/123/members, with body content {"member": "user B"}, then user B accepts the sharing through some ways, then both user A and user B can see and use that resource.

* User A created a resource and share it with user B, but user B doesn't want to accept it, and user B could reject and can't see and user that resource.