Descriptions and names of image_uri/image_id unclear

[rberlind]

While the top-level vars.tf has image_uri, the consul-cluster/vars.tf has image_id. The differences in names and various descriptions make it unclear what should be provided for this in the tfvars file. I first tried "consul-ubuntu-2018-02-02-222413" but had to change that to "/subscriptions/c0a607b2-6372-4ef3-abdb-dbe52a7b56ba/resourceGroups/roger-consul/providers/Microsoft.Compute/images/consul-ubuntu-2018-02-02-222413" which was what I found as the "Resource ID" of my image in Azure.