rally.deployment.credential.Credential class was a temporary workaround
to support multi-platfrom in Rally. It had not be anounced and a better
solution was found (spoiler: rally.env.platform.Platform [1][2]).
While CredentialBuilder was already removed (see [3]), Credential base
left for OpenStack support and some compatibility stuff.
Backward compatible logic can be reworked (and it is what this patch do)
in such way so we can remove th whole rally.deployment module without
breaking anyone.
[1] - Ide95c8b1e8e72293c009f4cd5e430e64eb1dd604
[2] - I83369e899a8b8609a8cd1e77b895d3a1ef8ae3e4
[3] - If7bde619f7c2e99248a071ff18b8084553e1bec0
Change-Id: I98d5c2e0113672b9f280d30f5550413a13f032ee
a16f53534b