snapshot_id is missing in ebs-configuration in nomad-cluster

[pp23]

To create an ebs_block_device AWS requires either a valid volume size or a snapshot id. So if no volume size or a volume size of 0 was provided the creation of ebs devices will result in an AWS error which that states that neither a volume size nor a snapshot id has been specified.

https://github.com/hashicorp/terraform-aws-nomad/blob/master/modules/nomad-cluster/main.tf#L83