fix: updated the DA to default to the "Default" resource group (#508)

* chore: update existing_resource_group_name variable format and catalog entries

* ensuring consistency

* ensuring consistency

Author: piyush117

ibm_catalog.json

@@ -109,15 +109,17 @@
             },
             {
               "key": "existing_resource_group_name",
-              "required": true,
+              "display_name": "resource_group",
               "custom_config": {
                 "type": "resource_group",
                 "grouping": "deployment",
                 "original_grouping": "deployment",
                 "config_constraints": {
                   "identifier": "rg_name"
                 }
-              }
+              },
+              "default_value": "Default",
+              "description": "The name of an existing resource group to provision the resources."
             },
             {
               "key": "existing_secrets_manager_instance_crn"