Add missing extend_target driver method

The base target driver class is missing the extend_target method which
is called from the LVM driver.

Since it was missing there some drivers, such as the nvmeof base class,
also missed it when they were implemented.

This patch adds a base implementation that does nothing, since this is
usually the right thing to do for most drivers.

Change-Id: If009af0a385d9203bd74e42e822dca299a119ca7

cinder/volume/targets/driver.py

@@ -85,3 +85,11 @@ class Target(object, metaclass=abc.ABCMeta):
         """
         a_host = A.get('host')
         return a_host and (a_host == B.get('host'))
+
+    def extend_target(self, volume):
+        """Reinitializes a target after the volume has been extended.
+
+        Most drivers don't need to do anything, but in other cases this may
+        cause IO disruption.
+        """
+        pass