Work without admin_token_auth middleware

Requests would fail with a 500 Internal Server Error if the
admin_token_auth middleware was removed from the paste pipeline.
The requests would fail because the code assumed that the context
contained an 'is_admin' element, but that element was only in
the context if the admin_token_auth middleware was in the
pipeline.

This change makes it so that if the admin_token_auth middleware
isn't in the paste pipeline requests will not fail with a
500 Internal Server Error.

Change-Id: Ic064785226ee70ee475d8f72fea3c2ae6971a07f
Fixes: bug 1190708
Merge "Work without admin_token_auth middleware"