Add btrfs support

[LKaemmerling]

Replaces #186

Signed-off-by: Lukas Kämmerling lukas.kaemmerling@hetzner-cloud.de

[fnkr]

I tried this today and it works great. Resizing works too.

[felixscheinost]

For some reason this isn't working for me. How are you supposed to use this functionality?

I created a new StorageClass with csi.storage.k8s.io/fstype: btrfs. When I try to create a pod that uses that storage class I get an error: MountVolume.SetUp failed for volume "pvc-eebbbe87-469b-4aba-a17c-9b080160051c" : rpc error: code = Internal desc = failed to publish volume: unsupported disk format btrfs

Looking for unsupported disk format I found FormatDisk in volumes/mount.go, where btrfs isn't handled yet. Is this an error? Do the disks need to be preformatted? Do I need to set some other parameter?