Add user/tenant shim to RequestContext

This is necessary because we pass the Nova context object to
openstack-common/code (logging, for example), which expects the
attributes to be named user/tenant not user_id/project_id.

Fixes bug 1098278

Change-Id: Ic0dc49ef54515fc0ffaa0895cf4d88701afb1e16
(cherry picked from commit ce098ccce9d67df8c243fa885640989a4703ac53)