Automatic merge from submit-queue (batch tested with PRs 55023, 65499). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Bugfix/csi default fs type

This PR address the issue mentioned in the following ticket https://github.com/kubernetes/kubernetes/issues/65122 
The FSType string will now not be defaulted to ext4. Removes defaulting of CSI file system type to ext4. CSI plugins that depended on this default need to be updated as the fsType would remain an empty string if not provided and would not default to ext4. CSI spec allows for an empty fstype string. This is intended for non-block plugins like nfs and gluster where filesystems are not separately created on the volume. But currently the default file system is overridden to ext4  which makes the above case redundant. This commit prevents such an overridding. 


```release-note  
ACTION REQUIRED: Removes defaulting of CSI file system type to ext4. All the production drivers listed under https://kubernetes-csi.github.io/docs/Drivers.html were tested and work as expected after this change. If you are using a driver not in that list, please test the drivers on an updated test cluster first. ```