Fix issues with Prometheus config generation

related to newly introduced merge mechanism.
1) Per-host overrides cannot be run_once.
2) Since merge_yaml is silent about missing files, it ignored
   the fact that no proper file was given due to wrong variable
   being referenced (see the closed bug).

Change-Id: I6db4af4c6e3364838bdae510f300038b0c1560b0
Closes-Bug: #1882460