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!
`uname` will not return what you think it returns here. It actually just tells you which kernel is currently running i.e. the one of the host system. That's the reason for PR #165. I suggest you simply copy & paste the [`apt-cache show` line](https://github.com/andsens/bootstrap-vz/pull/165/files#diff-c91d14cc00f5bde561ad389a0dfd3628R55). I'll probably generalize this further down the line.