Cleanup test_no_admin_token_auth cleanup code

The addCleanup call for the temporary paste config file was separated
from where it was needed because the function that required it
was outside the TestNoAdminTokenAuth class. This moves the
function to be a member and moves the addCleanup call in the
member function right where it's needed.

It's better to put the addCleanup code close to where it's needed
because then it will be called even if an exception occurs.

Change-Id: I50c97a3229958f4d15c7cd68ffa2a9d45914d1db
Merge "Cleanup test_no_admin_token_auth cleanup code"