Allow data bag names to be attribute-configurable; defaults are backwards compatible

This is a forward-port of a pull request submitted against 0.3.0 a while back. I think it makes sense as we manage multiple pools of squid proxies.
Prefer

```ruby
Chef::Log.info "no '#{node['squid']['hosts_data_bag_name']}' data bag"
```
because the `+` operator handles non-string values more poorly than just straight string interpolation.