Improvements to EC2 metadata handling

[tas50]

There's a few changes here so far.

Better logging when we reject a version that Amazon says they support
Drop our timeouts to a more reasonable time. These APIs are crazy fast there's no need to wait 30 seconds
Only setup the http client object once and use methods that allow connection reuse
Don't parse the best API version 3 times
Add all current EC2 metadata versions

[tas50]

@ViktorSchlaffer This should resolve #994 by only creating a single net/http object and using the appropriate methods that allow connection reuse within net/http. I'm not able to verify the change using AWS however since they only support http 1.0 w/o keepalives. Any chance you can pull this branch down somewhere and let me know if you see a reduction in connections?

[ViktorSchlaffer]

@tas50 I've verified you changes in our Openstack environment, and now the ec2 plugin uses only one tcp connection for the http requests. I have not been able to deploy this on all of our instances, as that decision is not in my hands, but from a functional point of view, this is what I was looking for, so I expect, that it will resolve the connection pool starvation issue.
Thanks for your help Tim!