Add root disk constraint

As clarified with William, this is the actual local, ephemeral disk in the instance, for which different flavors in EC2 have different values, and in openstack you can also create flavors with different values of local ephemeral disk. It is not related in any way to persistent storage.

Upon more investigation, the definition of this varies a bit by provider.

- For openstack, you can create custom flavors that have a default root disk size upon booting. The cloud image will resize the root filesystem to match the available root disk size.
- For ec2 ebs instances, which we default to using, the root disk size defaults to 8G but you can specify a custom root disk size via block device mapping. refs:
 - http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html
 - http://alestic.com/2009/12/ec2-ebs-boot-resize

Example:

  ec2-run-instances --key KEYPAIR --block-device-mapping /dev/sda1=:100 AMI

... creates an instance that boots with a created on-demand 100G ebs which gets thrown away when the instance is terminated. We'll need to implement passing the block device mapping in the goamz run instance api call to expose this.

- For azure, the os disk size is hardcoded and might make less sense to specify a constraint on disk size, but we should do it for consistency anyway. refs: http://msdn.microsoft.com/en-us/library/windowsazure/dn197896.aspx

Also worth noting:

Both ec2 and openstack have a concept of having a separate ephemeral disk attached to the instance as it boots.

For ec2 the number of attached disks and their size varies with instance types:

  http://aws.amazon.com/ec2/instance-types/

For openstack, you can define a single ephemeral disk of varying size, which is attached but not mounted by default afaict.

For azure, it seems like a default temporary disk 'D' is created but I can't find any information about its configurability or default size.

Handling ephemeral disk constraints should be implemented as a separate feature.