vagrant: support easily switching vm disk caching types
vagrant: support easily switching vm disk caching types

Add a top level global var to the Vagrantfile in order to easily
change the libvirt disk caching type. This can allow users to
choose their own perf/safety tradeoff on the VMs.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

gluster/gluster-kubernetes,gluster/gluster-kubernetes