EC2: support customization of mounted root volume. Handles size and volume type including provisioned IOPS

John;
This provides support for setting root volume size and type. It also includes a merge against the main branch, so if it's easier to cherry-pick here is the specific commit:

https://github.com/chapmanb/elasticluster/commit/043766a553a21a0daf0770d0907b10e20c492e53

Thanks much.