Fix binding l2-only ports by setting connectivity property

As NGS inherits from neutron-lib's api.MechanismDriver, `connectivity`
must be overridden, or it will return `portbindings.CONNECTIVITY_LEGACY`,
rather than what we want, `CONNECTIVITY_L2`.

Fixes-Bug: #2097803
Change-Id: I0c9ba7942d6e7c01f6ca77a49ae8fe447db30baf

Author: msherman64

networking_generic_switch/generic_switch_mech.py

@@ -32,6 +32,10 @@
 
 class GenericSwitchDriver(api.MechanismDriver):
 
+    @property
+    def connectivity(self):
+        return portbindings.CONNECTIVITY_L2
+
     def initialize(self):
         """Perform driver initialization.