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 
the `:enable_cross_zone_balancing` option is a `:boolean => true`, so using that when the option is not set results in the following hash being set:

    {"CrossZoneLoadBalancing" => nil}

Rather than always setting CrossZoneLoadBalancing, the condition will only set it if it is specified via the option, leaving it as-is without the parameter. However, negating the option isn't working. Seems like mixlib only introduced negated boolean options in 1.2.2 while chef uses 1.1. Committing a fix shortly.