Updated code and docs to reflect correct terminology

- Updated instances of fsid with clusterid
- Updated instances of credentials/subject with user/key

Signed-off-by: ShyamsundarR <srangana@redhat.com>

pkg/rbd/rbd_attach.go

@@ -280,7 +280,7 @@ func createPath(volOpt *rbdVolume, userID string, creds map[string]string) (stri
 	}
 
 	klog.V(5).Infof("rbd: map mon %s", mon)
-	key, err := getRBDKey(volOpt.FsID, userID, creds)
+	key, err := getRBDKey(volOpt.ClusterID, userID, creds)
 	if err != nil {
 		return "", err
 	}