[nginx] Fix argument order of getAttr

Summary:
@austin reversed the arguments of getAttr in a few places. This fixes
those so that we can use `upstreams` and `extraHttpConfig` in nginx.

Signed-off-by: Ricky Elrod <ricky@elrod.me>

Test Plan:
Ran this as part of my development cycle for working on the nagios role.
Also confirmed with [the manual](http://nixos.org/nix/manual/#ssec-builtins)
and saw `builtins.getAttr s set`.

Reviewers: johnw, austin

Reviewed By: austin

Subscribers: relrod, austin

Differential Revision: https://phabricator.haskell.org/D17