Add keystone v3 EC2 credential tests and client

The v2 version of the /users/{}/credentials/OS-EC2 API was already
present, for completeness this patch adds the v3 equivalent.

This API is not documented because it is preferred to use the
/v3/credentials API for this functionality.

Change-Id: Ide90658fa07a3d5a7ba2e52c3ab7377170273b76
4 files changed