Token trusts

 One issue that I've been asked about repeatedly is getting a token for an action in the future. Two use cases for this have come up:

1. HEAT and failover. It needs to move a virtual machine from one host to another.
2. Content production. Something generates a large file and needs to store it in swift.

In both cases, the users authorizes it at setup time to perform this action any time in the future, long after the token is expired.

To support this, add two new APIs. One is POST trust, and the other is GET trusts/{trust_id}

When POSTing to trusts, the user supplies a user that will be allowed to fetch a token at some point in the future.

When GETting tokens/trusts/{user_id} only the specified user will be able to fetch a token for the user that performed the trusted action.

We could potentially add an additional PATCH to modify a pre-auth arraingement. We would certainly want a DELETE.

The preauthentication id should be just a UUID. It should be useless to anyone but the user that creates it. No other user should be able to view it. The user should be able to enumerate her preauthentications, in order to view, modify, and delete them. /users/preauthentications