Hide resource visibility if user is not in defined group

[lukasfrank]

Proposed Changes

• Added pool-status-view-allowed-groups flag to api-server to define groups which can unhide the pool resources
• If pool-status-view-allowed-groups is empty every user can see pool resources

Fixes #813

[adracus]

Can we have this concept in a more extensible way?

• Currently, via this flag, all pools are affected - fine grained control is not possible / extending this to other resources requires lots of coding and flag-passing again
• The white-out happens after the fields are deserialized - can't we somehow enhance the individual codecs to respect custom tags in the struct definition to avoid iterating over the result objects again?

[afritzler]

@adracus how about using a SubResource for the pool capacity information? That way we can restrict the access via k8s RBAC and don't have to fiddle around in the Status on a per field level who is allowed to see what.