Limits yaml schema

[milliams]

This allows us to print:

[opc@mgmt ~]$ ./finish 
ERROR: Syntax error in `limits.yaml`.

if there is a limits.yaml like:

VM.Standard2.1:
1: 1
2: 1
3: 1
VM.Standard2.2:
1: 1
2: 1
3: 1

Fixes #28

[milliams]

This needs updating to support the different format we use for GCP.