Set discovery_default_driver=pxe_ipmitool and add missing release note

This is a follow-up to commit 350aeaff66.

1. Change the default driver for new nodes to pxe_ipmitool as it's our
   go-to default driver.

2. Add a release note with a link to more details.

Change-Id: I3535cbfbffd2ce394b4e828c53a7fba6ee46fb5c
Implements: blueprint inspector-node-autodiscovery

instack_undercloud/tests/test_undercloud.py

@@ -350,14 +350,14 @@ class TestGenerateEnvironment(BaseTestCase):
     def test_enabled_discovery(self):
         conf = config_fixture.Config()
         self.useFixture(conf)
-        conf.config(enable_node_discovery=True)
+        conf.config(enable_node_discovery=True,
+                    discovery_default_driver='foobar')
         env = undercloud._generate_environment('.')
         # The list is generated from a set, so we can't rely on ordering.
         # Instead make sure that it looks like a valid list by parsing it.
         drivers = json.loads(env['ENABLED_DRIVERS'])
-        # Discovery requires enabling the default driver, which is fake by
-        # default
-        self.assertEqual(sorted(drivers), ['fake', 'pxe_drac', 'pxe_ilo',
+        # Discovery requires enabling the default driver
+        self.assertEqual(sorted(drivers), ['foobar', 'pxe_drac', 'pxe_ilo',
                                            'pxe_ipmitool', 'pxe_ssh'])
         self.assertEqual(env['INSPECTION_NODE_NOT_FOUND_HOOK'], 'enroll')
 

instack_undercloud/undercloud.py

@@ -278,7 +278,7 @@ _opts = [
                       'information for newly enrolled nodes.')
                 ),
     cfg.StrOpt('discovery_default_driver',
-               default='fake',
+               default='pxe_ipmitool',
                help=('The default driver to use for newly discovered nodes '
                      '(requires enable_node_discovery set to True). This '
                      'driver is automatically added to enabled_drivers.')

releasenotes/notes/node-discovery-8264e0c97cb5e00f.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Allow enabling auto-discovery of ironic nodes by setting new option
+    ``enable_node_discovery=True`` in the ``undercloud.conf``. When enabled,
+    adds unknown nodes that boot the introspection ramdisk to ironic in the
+    ``enroll`` provisioning state and the driver set to the value of
+    ``discovery_default_driver`` configuration option (``pxe_ipmitool`` by
+    default). See ironic-inspector documentation for more details:
+    http://docs.openstack.org/developer/ironic-inspector/usage.html#discovery.

undercloud.conf.sample

@@ -141,7 +141,7 @@
 # The default driver to use for newly discovered nodes (requires
 # enable_node_discovery set to True). This driver is automatically
 # added to enabled_drivers. (string value)
-#discovery_default_driver = fake
+#discovery_default_driver = pxe_ipmitool
 
 # Whether to enable the debug log level for Undercloud OpenStack
 # services. (boolean value)