pkg/types: Add DefaultMachinePlatform
pkg/types: Add DefaultMachinePlatform

Providing a fallback here allows you to avoid repeating the same
config across several machine pools.  For example, you may want to use
the same AWS instance type for both masters and workers.

Also improve the LibvirtPlatform.URI comment to make it easier to
distinguish from installer/pkg/config/libvirt's Libvirt.Image.  That
image location currently has no analog in the InstallConfig structure.
It was removed in 9ee45fb6 (*: provide a default OS image for libvirt,
2018-09-17, #271), but we may restore it soon.

derekhiggins/installer,derekhiggins/installer