Do not remove ``haproxy`` package

The principal charm may have use for it.

Change-Id: I342e3be7067a218c466a94728920377367239887
Closes-Bug: #1832739

hooks/neutron_ovs_hooks.py

@@ -177,7 +177,9 @@ def neutron_plugin_joined(relation_id=None, request_restart=False):
         #       in use as this will remove the l3 agent
         #       see https://pad.lv/1515008
         if not use_dvr():
-            pkgs.extend(METADATA_PACKAGES)
+            # NOTE(fnordahl) do not remove ``haproxy``, the principal charm may
+            # have use for it. LP: #1832739
+            pkgs.extend(set(METADATA_PACKAGES)-set(['haproxy']))
         purge_packages(pkgs)
     secret = get_shared_secret() if enable_nova_metadata() else None
     rel_data = {

unit_tests/test_neutron_ovs_hooks.py

@@ -216,8 +216,7 @@ class NeutronOVSHooksTests(CharmTestCase):
             **rel_data
         )
         self.purge_packages.assert_called_with(['neutron-dhcp-agent',
-                                                'neutron-metadata-agent',
-                                                'haproxy'])
+                                                'neutron-metadata-agent'])
         self.assertFalse(self.install_packages.called)
 
     @patch.object(hooks, 'os_release')