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
diff --git a/tempest/common/cred_provider.py b/tempest/common/cred_provider.py
index 033410e..c22dc1f 100644
--- a/tempest/common/cred_provider.py
+++ b/tempest/common/cred_provider.py
@@ -113,3 +113,7 @@
     @abc.abstractmethod
     def is_multi_tenant(self):
         return
+
+    @abc.abstractmethod
+    def get_creds_by_roles(self, roles, force_new=False):
+        return