Fix problem propagating the "product_version" attribute

[meaksh]

What does this PR change?

I've noticed there was a problem where the product_version was not set properly on the deployed nodes when not having a product_version definition in the "base" module but instead inside each different nodes.

Like in this example:

module "base" {
[... no product_version defined here ...]
}

module "suma-43-srv" {
  source = "./modules/server"
  base_configuration = module.base.configuration

  name = "suma-43-srv"
  product_version = "4.3-nightly"
}

module "suma-43-min-build" {
  source = "./modules/build_host"
  base_configuration = module.base.configuration

  name = "suma-43-min-build"
  product_version = "4.3-released"
  server_configuration = module.suma-43-srv.configuration
  image = "sles15sp4o"
}

module "suma-43-min-host" {
  source = "./modules/host"
  base_configuration = module.base.configuration

  name = "suma-43-min-host"
  image = "sles15sp4o"
}

Before this PR, i.a. the suma-43-srv node was not deploying correctly, as it got product_version = "" and therefore not installing the SERVER stack.

This PR should fix this problem and use the product_version from the node if defined, if note take the one from "base" module.

[srbarrios]

Thank you!

[maximenoel8]

LGTM, I missed getting the variable to backend host. Sorry .