[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)

OS-14200

doc/source/user/index.rst

@@ -1108,7 +1108,7 @@ Network driver (network-driver)
   `Networking`_ section for more details.
 
 Volume driver (volume-driver)
-  Specified in the ClusterTemplate to select the volume driver.  The supported
+  Specified in the ClusterTemplate to select the volume driver. The supported
   volume driver is 'cinder', allowing Cinder volumes to be mounted in
   containers for use as persistent storage.  Data written to these volumes
   will persist after the container exits and can be accessed again from other
@@ -1116,10 +1116,12 @@ Volume driver (volume-driver)
   will be deleted.  Refer to the `Storage`_ section for more details.
 
 Storage driver (docker-storage-driver)
-  Specified in the ClusterTemplate to select the Docker storage driver.  The
+  Specified in the ClusterTemplate to select the Docker storage driver. The
   default is 'devicemapper'. Refer to the `Storage`_ section for more
   details.
 
+  **NOTE:** For Fedora CoreOS driver, devicemapper is not supported.
+
 Image (image)
   Specified in the ClusterTemplate to indicate the image to boot the servers.
   The image binary is loaded in Glance with the attribute
@@ -2900,6 +2902,12 @@ of Docker storage drivers available.
   container isolation, although it still runs in enforcing mode on the
   cluster compute instances.
 
+* 'overlay2' is the preferred storage driver, for all currently supported
+  Linux distributions, and requires no extra configuration. When possible,
+  overlay2 is the recommended storage driver. When installing Docker for
+  the first time, overlay2 is used by default.
+
+
 Persistent storage
 ------------------