Use m3.medium instead i3.large
Use m3.medium instead i3.large

Avoids:
```
15:12:06 | Preparing deployment: Preparing deployment (00:00:00)
15:12:06 | Creating missing vms: raw-ephemeral-disk-test/aa7b627d-8a2d-4681-bcde-8925a2e90179 (0) (00:11:00)
            L Error: Timed out pinging to a88947da-0298-4c3d-a7b7-3a1f0bfdc956 after 600 seconds

15:23:06 | Error: Timed out pinging to a88947da-0298-4c3d-a7b7-3a1f0bfdc956 after 600 seconds
```

The fix will be done as part of this story: https://www.pivotaltracker.com/story/show/142878939

[#142858319](https://www.pivotaltracker.com/story/show/142858319)

Signed-off-by: Chris Dutra <7eb54fc324f04033595eb420f2ee7c112fb7259e@pivotal.io>

cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release