Implement "BaseCommand" result property

``ovsdbapp.api.Command`` abstract class will declare the "result"
property in [1]. This should be available in the next library
release.

Related-Bug: #1995078

[1]https://review.opendev.org/c/openstack/ovsdbapp/+/871836/11/ovsdbapp/api.py

Change-Id: I98fd0be0b7d3ca8dd631b00b75515b1ad0d61f3c

vif_plug_ovs/ovsdb/impl_vsctl.py

@@ -116,6 +116,15 @@ class BaseCommand(ovsdb_api.Command):
         self.cmd = cmd
         self.opts = [] if opts is None else opts
         self.args = [] if args is None else args
+        self._result = None
+
+    @property
+    def result(self):
+        return self._result
+
+    @result.setter
+    def result(self, value):
+        self._result = value
 
     def execute(self, check_error=False, log_errors=True):
         with Transaction(self.context, check_error=check_error,