Fix net_config_override validations

There is an issue with the _validate_no_ip_change function
where it doesn't work correctly with a custom net_config_override
file.

This patch resolves validation issue so that custom os-net-config
templates are taken into account. Additionally it makes it so
that the net_config_override can be empty, thus providing the
user a means of disabling os-net-config if they choose to do so.

Change-Id: Iada099a6bddc7927d26b19776e705b1ac231b2e5
Depends-On: I69eb812af2d653cb3eb29694ae29b57ebb2de53d
Closes-bug: #1764507

instack_undercloud/tests/test_undercloud.py

@@ -379,6 +379,15 @@ class TestNoIPChange(BaseTestCase):
         mock_loads.return_value = mock_config
         undercloud._validate_no_ip_change()
 
+    @mock.patch('instack_undercloud.undercloud.open')
+    @mock.patch('os.path.isfile', return_value=True)
+    def test_update_empty(self, mock_isfile, mock_open):
+        # This would be a way to disable os-net-config from running
+        mock_open.side_effect = [
+            mock.mock_open(read_data='').return_value,
+        ]
+        undercloud._validate_no_ip_change()
+
     @mock.patch('instack_undercloud.undercloud.open')
     @mock.patch('json.loads')
     @mock.patch('os.path.isfile', return_value=True)

instack_undercloud/undercloud.py

@@ -818,16 +818,22 @@ def _validate_no_ip_change():
     need to disallow it early in the install before configurations start to
     be changed.
     """
-    os_net_config_file = '/etc/os-net-config/config.json'
+    if CONF.net_config_override:
+        os_net_config_file = CONF.net_config_override
+    else:
+        os_net_config_file = '/etc/os-net-config/config.json'
     # Nothing to do if we haven't already installed
     if not os.path.isfile(
             os.path.expanduser(os_net_config_file)):
         return
-    with open(os_net_config_file) as f:
-        network_config = json.loads(f.read())
     try:
+        with open(os_net_config_file) as f:
+            network_config = json.loads(f.read())
         ctlplane = [i for i in network_config.get('network_config', [])
                     if i['name'] == 'br-ctlplane'][0]
+    except ValueError:
+        # File was empty
+        return
     except IndexError:
         # Nothing to check if br-ctlplane wasn't configured
         return

undercloud.conf.sample

@@ -120,9 +120,8 @@
 
 # Path to network config override template. If set, this template will
 # be used to configure the networking via os-net-config. Must be in
-# json format. Templated tags can be used within the template, see
-# instack-undercloud/elements/undercloud-stack-config/net-
-# config.json.template for example tags (string value)
+# json format. If you wish to disable os-net-config you can use an
+# set this location to point to an empty file.
 #net_config_override =
 
 # Network interface on which inspection dnsmasq will listen.  If in