Make ec2 default to an "all-open" sec group

Summary:
This depends on an `all-open` security group existing, but it seems better and
more flexible to let the individual machines handle their own firewall.

Test Plan:
This is what I was using earlier when @austin and I were talking on
IRC.

Reviewers: johnw, austin

Reviewed By: austin

Subscribers: austin

Differential Revision: https://phabricator.haskell.org/D10