Fix keyerror in stemcell fetch command

tile_generator/opsmgr.py

@@ -690,6 +690,6 @@ def get_stemcells():
 	response = get('/api/v0/diagnostic_report', check=False)
 	if response.status_code == requests.codes.ok:
 		diag = response.json()
-		stemcells = diag['stemcells'] or diag['available_stemcells'] or []
+		stemcells = diag.get('stemcells') or diag.get('available_stemcells') or []
 		return stemcells
 	return []