Add a role for EmrEtlRunner and StorageLoader

We recently followed the EmrEtlRunner and StorageLoader [setup instructions](https://github.com/snowplow/snowplow/wiki/1-Installing-EmrEtlRunner) and created this Ansible role to codify our setup. We used ubuntu-trusty-14.04 on EC2 (ami-864d84ee).

We thought we'd share this so that future Snowplow users can use it. Is this a good place to share it?