add vtx.rb fact in openstack-full

vtx fact is used to detect if VTX feature is enabled or not on compute
nodes.
While the pull-request is merged upstream in puppetlabs/facter, we add
the fact in the openstack-full role manually.
This patch will be reverted once the patch upstream merged.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>