Add support for enhanced networking on EC2

Hi all,

I've added support as a plugin for EC2's enhanced networking using the new Intel driver. The steps in the plugin follow the [official EC2 documentation](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/enhanced-networking.html) for enabling it.

The existing enhanced networking installation task wasn't used (and didn't work), so I've also removed that.

On my test instances, enhanced networking reduces latency by 10-20% and jitter by almost 90%.

This feels like something that should be automatically enabled for EC2/HVM configurations, so I'd be happy to refactor it to the EC2 provider as well. I believe Ubuntu enables it by default for HVM.

Thanks!