prefix aws_ to variables in global scope

This change was made because in some contexts, this file is included from scopes which define and rely on variables such as $path. To the extent that we need to use the global space for variables, this change attempts to ensure that at least they are prefixed with an appropriate string that should reduce conflicts with calling scopes. 

(specific example is Drupal 7, which relies on a $path variable that without this change is unset() on line 1516)