Look for heat data files in /var/lib/heat-cfntools

The /var/lib/heat-cfntools directory should be owned by the
heat-cfntools package for whichever distro it is included.

This avoids the problem of heat writing to directories owned
by cloud-init.

This is the guest-side change which corresponds to this review
https://review.openstack.org/#/c/23052/

Fixes: Bug #1105806