get context from req rather than getting a new admin context

This patch removes the call to context.get_admin_context() and instead
obtains the context from the req object. Allowing the context.auth_token
to be reused.

Fixes bug 1161081

Change-Id: I3b104edc2966fc3263fad824b1c4c99df832672e
Merge "get context from req rather than getting a new admin context"