correct typo of config option name in error message

Change-Id: I146e6ac742649ec24c6fbfd8b73a5359e447e76f
Use admin_prefix consistently

The auth_admin_prefix config option is used manually in a number of
places where it could just be set for all requests.

Change-Id: Ib690d69bdf075cf7095841a6ab9f419dfe8b1ca8
Merge "Use admin_prefix consistently"
Merge "correct typo of config option name in error message"