Fix gcp memory utilization

[eclisson-clt]

Update/Fix du detector sur l'utilisation mémoire

[haedri]

I think it can be problematic since thresholds are not the same at all (usage % vs free %), for example if previous variable memory_utilization_threshold_critical was manually set, then updating will trigger this detector a lot

2 solutions :

1. Change signal from free % to usage % (100 - signal), impacts will be minimal for users
2. Change filename memory_utilization so that variable name changes also

Solution 2 might be easier I think