Rehome provider_network_attribute_updates_supported to API definition

Rehomed provider_network_attribute_updates_supported to
plugins.ml2.api.MechanismDriver, in order to be inherited in other
mechanism drivers.

Change-Id: I145861b0ca356e3e27495fb2b0205f6e8339a964
Closes-Bug: #1823865

neutron_lib/plugins/ml2/api.py

@@ -447,6 +447,17 @@ class MechanismDriver(object):
         """
         return False
 
+    @staticmethod
+    def provider_network_attribute_updates_supported():
+        """Returns the provider network attributes that can be updated
+
+        Possible values: neutron_lib.api.definitions.provider_net.ATTRIBUTES
+
+        :returns: (list) provider network attributes that can be updated in a
+                         live network using this driver.
+        """
+        return []
+
 
 @six.add_metaclass(abc.ABCMeta)
 class _TypeDriverBase(object):

releasenotes/notes/rehome-provider-network-attribute-updates-supported-ea02a526ef297053.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - The function ``provider_network_attribute_updates_supported`` is now
+    available in ``neutron_lib.plugins.ml2.api.MechanismDriver``.