fix(ec2): add explicit 100GB root volume

By default the m3 instances (of all types) use an 8GB root volume.
The SSD volume is unused and located at /dev/xvdb. This PR changes
the 8GB root volume to 100GB, providing enough storage for a basic
Deis cluster.

fixes #882
Merge pull request #895 from deis/fix-ec2-space

fix(ec2): add explicit 100GB root volume