Add 'cloudware-install' target to handle copying cloud
disk images to a directory within DESTDIR.

Add CLOUDINSTALL variable to automatically generate the
actual list of install targets. While here, rename the
cloud-specific names from 'vm-' to 'cw-' to avoid naming
collisions.

Add AZURE_DISK, GCE_DISK, and OPENSTACK_DISK output file
variables.

Add 'cloudware-install' to the 'install' target.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation