fix(topokey): changing topology key to unique name

There are setups where nodename is different than the hostname.
The driver uses the nodename and tries to set the "kubernetes.io/hostname"
node label to the nodename. Which will fail if nodename is not same as
hostname. Here, changing the key to unique name so that the driver can set
that key as node label and also it can not modify/touch the existing node labels.

Now onwards, the driver will use "openebs.io/nodename" key to set the PV node affinity.
Old volumes will have "kubernetes.io/hostname" affinity, and they will also work as
after the PR openebs#94, it supports all the node
labels as topology key and all the nodes have "kubernetes.io/hostname" label set. So
old volumes will work without any issue. Also for the same reason old stoarge classes
which are using "kubernetes.io/hostname" as topology key, will work as that key is supported.

Signed-off-by: Pawan <pawan@mayadata.io>

pkg/zfs/volume.go

@@ -42,7 +42,7 @@ const (
 	// ZFSNodeKey will be used to insert Label in ZfsVolume CR
 	ZFSNodeKey string = "kubernetes.io/nodename"
 	// ZFSTopologyKey is supported topology key for the zfs driver
-	ZFSTopologyKey string = "kubernetes.io/hostname"
+	ZFSTopologyKey string = "openebs.io/nodename"
 	// ZFSStatusPending shows object has not handled yet
 	ZFSStatusPending string = "Pending"
 	// ZFSStatusReady shows object has been processed