Add support for roles to credentials providers

This commit adds support for requesting credentials by a role from the
credentials providers. This entails 2 things, first adding support for
specifying roles in a test-accounts file with test-accounts based
providers and secondly adding support to isolated-creds to assign
arbitrary roles on credentials requested by role.

Change-Id: I6a0f0539d41e0cf3d04414964b289447194d2793
Partially-implements: bp test-accounts-continued
7 files changed