Add "connectivity" property to "MechanismDriver"

This property will return the type of the driver: "l2", "l3" or
"legacy".

bp boot-vm-with-unaddressed-port
Related-Bug: #1959125
Related-Bug: #1821058

Change-Id: I5730c68e4e860028efa0541b0098c439c14524d7

neutron_lib/plugins/ml2/api.py

@@ -14,6 +14,8 @@
 
 import abc
 
+from neutron_lib.api.definitions import portbindings
+
 
 # The following keys are used in the segment dictionaries passed via
 # the driver API.
@@ -469,6 +471,16 @@ class MechanismDriver(object, metaclass=abc.ABCMeta):
         """
         return extensions
 
+    @property
+    def connectivity(self):
+        """Return the mechanism driver connectivity type
+
+        The possible values are "l2", "l3" and "legacy" (default).
+
+        :returns: a string in ("l2", "l3", "legacy")
+        """
+        return portbindings.CONNECTIVITY_LEGACY
+
 
 class _TypeDriverBase(object, metaclass=abc.ABCMeta):
 

releasenotes/notes/mechanismdriver-connectivity-00dc679a3f307345.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Add ``connectivity`` property to ``MechanismDriver``. For more information,
+    see
+    [`Boot a VM with an unaddressed port <https://blueprints.launchpad.net/nova/+spec/boot-vm-with-unaddressed-port>`_].