switch from 'zones' to 'instances'

It's more common (at least for users in AWS) to refer to cloud-based VMs as 'instances', which covers KVM, Xen and OS-virtualized types. I think this will make the tool even more useful to audiences not deeply familiar with Illumos/Solaris.

Big thanks for the tool, also. This makes it much easier to have teams self-motivated by cost savings :)