Load AWS information from a file.

Flags show up in the process name and in the `/debug/vars` of `expvar`. Using flags for secrets turns out to be a bad idea, thus the config file.

Alternative would have been to use environment variables: ops prefers config files.

## Question

- Should I move all the flags to a config file?

r: @camilo @fbogsany @shuhaowu 
cc: @Shopify/webscale 
Not a strong opinion but I think if we just use `BucketConfig`, it would be slightly more readable.