Unset vars for missing elements

If certain files are missing, don't export vars with bogus values like `cert-*`. You may not, for instance, need to use EC2 for all of your AWS accounts, and additionally [EC2_CERT and EC2_PRIVATE_KEY and their associated options for the EC2 CLI tools are deprecated][1].

If these variables indeed will not be exported, unset them so you can switch between accounts without tainting the environment with mixed account settings.

This branch also includes a commit that moves the gitkeep files to hidden -- prettier for *nix systems and it avoids having to work around them with `grep -v` and such. I can extract this commit though if you'd rather consider it separately.

[1]: http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/CLTRG-common-args-api.html