Explicitely type tag map variables in oke submodule (#709)

When trying to update defined_tags on a cluster, via the configuration:
```
  defined_tags = {
    oke = {
      cluster = {
        "NEW_KEY" = "NEW_TAG"
      }
      persistent_volume = {
        "NEW_KEY" = "NEW_TAG"
      }
      service_lb = {
        "NEW_KEY" = "NEW_TAG"
      }
      node_pool = {
        "NEW_KEY" = "NEW_TAG"
      }
      node = {
        "NEW_KEY" = "NEW_TAG"
      }
    }
  }
```

one gets these errors for the cluster, persistent_volume, service_lb parts:
```
defined_tags  = lookup(var.defined_tags, "<PART>", {})
Invalid value for "default" parameter: the default value must have the same type as the map elements.
```

this is because terrafrom cannot reconciliate the map string->string type of the tags submaps
with the type of the empty map.

Fix this as outlined in hashicorp/terraform#27509
by explicitely typing the submap variables.

node_pool and node do not suffer the same issue, as I assume the merge function does cleverer
overload resolution, but more precision doesn't hurt.

Signed-off-by: Andrea Stacchiotti <[email protected]>

Author: 12345ieee

modules/oke/variables.tf

@@ -133,9 +133,9 @@ variable "worker_nsgs" {
 }
 
 variable "freeform_tags" {
-  type = map(any)
+  type = map(map(string))
 }
 
 variable "defined_tags" {
-  type = map(any)
+  type = map(map(string))
 }