Allow for VPC boot disk sizes greater than 250GiB

Currently, VPC boot disks are limited to a max of 250GiB. We have use-cases where having larger boot disk sizes (e.g. 512GiB or 1TiB) would be adventageous. Other cloud providers that we have tested, allow for boot disk sizes in the mulitples of TiB. By allowing larger boot disk sizes, IBM Cloud would be keeping pace with its competitors.