Performance fixes for running ec2tag in non-aws environments

[hairyhenderson]

Fixes #110

This does two things:

1. caches the InstanceDescriber in case describer is called multiple times, and caches it earlier so that the e.metaClient.nonAws bool can be set earlier, avoiding unnecessary calls.
2. gives the EC2 client an http.Client with a 500ms timeout

On my mac, this reduces the amount of time it takes to timeout a ec2tag function from 900ms to 500ms.

And in Linux, it's down from 21s to 500ms.

Signed-off-by: Dave Henderson dhenderson@gmail.com