[cern][k8s] Fix docker storage of Fedora CoreOS
upstream: https://review.opendev.org/c/openstack/magnum/+/718296
In commit I1a75f1bf12747508a3497293650d3cc668202de6 the worker node is missed to add the docker storage support. And the current systemd unit is not really working. So this patch fixes it by removing the hardcode for /dev/vdb and using xfs instead of ext4 (the same way for Fedora Atomic) to make it simpler and solid.
Task: 39331 Story: 2005201
Change-Id: I4c465664eb19f1992df95750dd7b2d99688c6cae (cherry picked from commit c2439ca1)