(FACT-629) Don't use proxy in `reachable?` method of ec2 facts

This commit fixes an issue in which ec2 facts could not be
resolved with a proxy because the `reachable?` method did not
set `:proxy => nil` by default, causing the facts to be considered
unsuitable.
Merge pull request #766 from whopper/ec2_proxy_fix

(FACT-629) Don't use proxy in `reachable?` method of ec2 facts