feat(contrib/ec2): add m3 instances to template

The current generation of EC2 instances (M3) is faster and cheaper
than the old M1 instances. It's a more reasonable default to use
m3.medium as the default instance size.

fixes #689
fix(contrib/ec2): bump size to m3.large

A 5GB root volume is too small. We might run into BTRFS issues
with low disk space.
Merge pull request #768 from opdemand/add-m3-instances

feat(contrib/ec2): add m3 instances to template