Retry functionality fix

related to #24

This patch fixes a number of issues with regards to retry functionality:

1. last retry wiping out responce body, which resulted in no error reported
2. retry interval was too small (100 ms), increased to 500 ms avg
3. not enough retries
4. No randomness in exp backoff

We had a long history of problems while baking 10 AMIs in a time, this fix
along with some fix in packer resolves the issues