fix not working admin_auth_enabled=false on stats request.
add riak_test for admin_auth_enabled.
refactor admin authentication on users & stats wm resources.
change usage's query string to enlarge coverage on admin bypass test per review.
Merge pull request #854 from basho/bugfix/cs719-fix-admin-auth-enabled-on-stats

Fix not working admin_auth_enabled=false on stats request.

Reviewed-by: kuenishi