Prevent default silo from showing up in utilization response

[askfongjojo]

From the comment in #4943:

I've updated the /v1/system/utilization/silos endpoint to only return non-discoverable silos if they have a quota set. I believe the default-silo does get a quota set currently which is non-ideal, but that should be the only one that shows up on the list.

The default silo currently has all 9s for their quotas by default so they do show up in the Utilization UI. New customers will see them the first time they get on the rack.

[david-crespo]

We may want to handle this client-side if the silo in question has a well-known name.

[augustuswm]

It looked like the default-silo also affected the silo utilization totals, which largely skews them.