Use module functions to avoid method name conflict

Fixes OHAI-489: http://tickets.opscode.com/browse/OHAI-489

Fixes conflict over method names `fetch_metadata` and `http_client` in
the Ohai::System instance by modifying the GCE mixin to define these as
module functions and changing the plugin to call them as module
functions.
Merge branch 'OHAI-489'