CMK-33267: send the customer field on host-group creation in ultimatemt

The ultimatemt edition requires a customer on config objects, so a bare
host-group POST is rejected:

    400 ... "The customer field is required in this edition"

HostGroupsAPI.create was the one caller-facing group helper still posting
without it; users.create and passwords.create already guard the same way.
Follow that idiom so the helper works in every edition.

Fixes CMK-37915.

Change-Id: Icdf7c083092a1144af164e25687b4cef3df0f8c6

Author: rene-slowenski-checkmk

tests/testlib/openapi_session.py

@@ -849,9 +849,14 @@ def rename_and_wait_for_completion(
 
 class HostGroupsAPI(BaseAPI):
     def create(self, name: str, alias: str) -> requests.Response:
+        body = {"name": name, "alias": alias}
+        # In the ultimatemt edition every config object belongs to a customer,
+        # so the field is mandatory here.
+        if self.session.site_edition.is_ultimatemt_edition():
+            body["customer"] = "global"
         response = self.session.post(
             "/domain-types/host_group_config/collections/all",
-            json={"name": name, "alias": alias},
+            json=body,
         )
         if response.status_code != 200:
             raise UnexpectedResponse.from_response(response)