iSCSI: adjust to change in how blivet passes auth info

When blivet switched from using libiscsi to storaged, the node
attribute of an `iScsiDiskDevice` changed from being a libiscsi
`PyIscsiNode` instance to a blivet `NodeInfo` namedtuple, but
both blivet and anaconda continued trying to call the `getAuth`
method of the node - which `NodeInfo` doesn't have - to find
the authentication info for the node. I've sent a blivet PR
to store this information in the `NodeInfo` namedtuple instead:
storaged-project/blivet#518 . This patch
adjusts anaconda to use it.

pyanaconda/ui/gui/spokes/storage.py

@@ -418,14 +418,12 @@ def _create_iscsi_data(self, device):
         if self.storage.iscsi.ifaces:
             iscsi_data.iface = self.storage.iscsi.ifaces[dev_node.iface]
 
-        auth = dev_node.getAuth()
-        if auth:
-            if auth.username and auth.password:
-                iscsi_data.user = auth.username
-                iscsi_data.password = auth.password
-            if auth.reverse_username and auth.reverse_password:
-                iscsi_data.user_in = auth.reverse_username
-                iscsi_data.password_in = auth.reverse_password
+        if dev_node.username and dev_node.password:
+            iscsi_data.user = dev_node.username
+            iscsi_data.password = dev_node.password
+        if dev_node.r_username and dev_node.r_password:
+            iscsi_data.user_in = dev_node.r_username
+            iscsi_data.password_in = dev_node.r_password
         return iscsi_data
 
     @property