Create an scaleway_instance_image data source

[QuentinBrosse]

As it is described in #225, this data source will replace scaleway_image.

We have to take https://github.com/terraform-providers/terraform-provider-scaleway/issues/95 in consideration for the design of this new data source.

Proposal draft

data "scaleway_instance_image" "ubuntu" {
  architecture = "arm" # Desired arch. 
  label = "ubuntu-bionic" # Exact label of desired image.
  fixed = true # If true, never update the underlying local image UUID. 
}

[jerome-quere]

We also take the opposit approch:

data "scaleway_instance_image" "ubuntu" {
  architecture = "arm" # Desired arch. 
  label = "ubuntu-bionic" # Exact label of desired image.
  latest = true # If true will alwys fetch the latest version
}