(feature request) allow multiple credentials per user for phased credential rotation

[Ben Hartshorne]

In order to change the credentials for a running application, it is useful to be able to have multiple active credentials for a given user account. It allows the following procedure for credential rotation:

* create new credential
* change client configuration to use new credential
* revoke old credential

In large production enviromnents, it is often infeasible to change all the clients at once, requiring a slow shift from the old to the new credential. Amazon describes this approach here: http://docs.amazonwebservices.com/AWSSecurityCredentials/1.0/AboutAWSCredentials.html#CredentialRotation

[Dolph Mathews (dolph)]

This is now possible with the /v3/credentials API introduced in Grizzly, along with pluggable authentication (also new in Grizzly).