make image_id optional for VM disk

[treywelsh]

A terraform user suggested to make all arguments of template disk schema optional (for now, only image_id is required).

Here is the original message:
The idea behind this is to have a map that can be define as example:

disk {}

or

disk {
image_id = null
size = 0
}

that return {} too without error during instance creation process. I think that will give use more flexibility on vm creation