Add an 'is_admin' value to the keystone_user dict

In the actions we often would like a slightly different workflow path option when executed by an admin. Rather than make a different action, we could have the actions simply include those checks.

The problem is, how do we define what an admin is when role names are deployer configured. By default people tend to use "admin" but that isn't quite safe to assume. Therefore we should add a configurable is_admin check that runs in the middleware and adds a is_admin boolean to the keystone_user dict.

This means we can have that check do what we need to confirm someone is admin (and allow deployers to configure that), and then in code we can use that.