Allow modification of ELB attributes

Add a module that allows modification of ELB attributes. 

* Cross zone load balancing
* Connection draining (requires fog 1.23.0)
* Connection settings/Idle timeouts - not available until https://github.com/fog/fog/pull/3259 is merged

Fixes #4 
Can we skip the if check here? and use
```ruby
 attrs["CrossZoneLoadBalancing"] = {'Enabled' => config[:enable_cross_zone_balancing] }
```