release/Makefile.vm: Add support for CW flavours

There are two "Vagrant" images right now: "Vagrant Image for VirtualBox"
and "Vagrant Image for VMWare". Rather than listing these separately in
a CLOUDWARE list, place "VAGRANT" into a CLOUDWARE_TYPES list and then
use a VAGRANT_FLAVOURS variable to identify the two versions. Add make
logic to allow defaults (in this case, image format and filesystem) to
be specified once for VAGRANT and inherited by both flavours.

This will make future work to add flavoured EC2 images simpler.

Approved by: re (gjb)
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D41790

(cherry picked from commit e6eeac9b71795252b12a6b94e7a06633a9ff6e4e)
(cherry picked from commit fec4f6a7c377f576c78dabe16bc632ecf20058b4)