[ML] Use ml.machine_memory as sneaky way to assert ML node (#79622)

The way we detect an ML node from node attributes is now
to look for ml.machine_memory instead of ml.max_open_jobs.

Relates #79518

Author: droberts195

x-pack/plugin/src/yamlRestTest/resources/rest-api-spec/test/ml/jobs_get_stats.yml

@@ -103,7 +103,7 @@ setup:
   - match:  { jobs.0.state: opened }
   - is_true:  jobs.0.node.name
   - is_true:  jobs.0.node.transport_address
-  - match:  { jobs.0.node.attributes.ml\.max_open_jobs: "512"}
+  - is_true:  jobs.0.node.attributes.ml\.machine_memory
   - is_true:  jobs.0.open_time
   - match:  { jobs.0.timing_stats.job_id: job-stats-test }
   - match:  { jobs.0.timing_stats.bucket_count: 1 }  # Records are 1h apart and bucket span is 1h so 1 bucket is produced