Retry downloading update if update file is missing

>This change is for https://www.pivotaltracker.com/story/show/65969536

>When Tariff runs in multi-server environment one server may have downloaded the update and marked as pending. If other server tries to apply it, it won't find the file. Therefore we try to redownload the update if file is missing.

>It would be better to have a shared file storage like S3 though.

This is a rebased version of PR #148