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 
I don't think any of them really will be that clear. We don't really need a comment, but just reading that code means I need to take like a minute or two to think about exactly what's going on.

We could make 0077 a constant?