pvsadm with a private cos endpoint

[mkumatag]

There is requirement of using the private cos endpoint instead for the secure private n/w use case, currently I see an option --bucket-region to supply the region, this option is used internally to build the cos URI, this task is to supply the private. prefix with this flag and make sure things are working fine(upload and import)

e.g: Public EP: s3.us-south.cloud-object-storage.appdomain.cloud Private EP: s3.private.us-south.cloud-object-storage.appdomain.cloud

/cc @bpradipt /assign @bkhadars