Disable federated buckets when etcd is namespaced (#8709)

This is to ensure that when we have multiple tenants
deployed all sharing the same etcd for global bucket
should avoid listing each others buckets, this leads
to information leak which should be avoided unless
etcd is not namespaced for IAM assets in which case
it can be assumed that its a federated setup.

Federated setup and namespaced IAM assets on etcd
is not supported since namespacing is only useful
when you wish to separate the tenants as isolated
instances of MinIO.

This PR allows a new type of behavior, primarily
driven by the usecase of m3(mkube) multi-tenant
deployments with global bucket support.

cmd/globals.go

@@ -212,6 +212,10 @@ var (
 	// Allocated etcd endpoint for config and bucket DNS.
 	globalEtcdClient *etcd.Client
 
+	// Is set to true when Bucket federation is requested
+	// and is 'true' when etcdConfig.PathPrefix is empty
+	globalBucketFederation bool
+
 	// Allocated DNS config wrapper over etcd client.
 	globalDNSConfig *dns.CoreDNS