Add config to enable/disable read/writes to non-default buckets

[jreadey]

For enhanced security it would be best if the ability to read or write to an arbitrary bucket can be controlled using config settings. E.g. add to config.yml: allow_any_bucket_read: true allow_any_bucket_write: false

On AWS or Azure it's certainly possible to do the equivalent based on policy settings, but the above config will be a good default for those who inadvertently use credentials that allow read/write access to any bucket in their account.

[jreadey]

This is checked into master. The default config is: allow_any_bucket_read: true allow_any_bucket_write: true

for backward compatibility since I didn't want anyone to have mysterious failures with the update.