Populate plugin directory to fix port operation in nsxadmin v3

Following I7331e914234c5f0b7abe836604fdd7e4067551cf, nsxadmin command
"nsxadmin -o list-mismatches -r ports" failed due to absence of core plugin
in plugin directory. Fix this by adding core plugin from the admin code

Change-Id: I4edbe0b413a66c65c7061dee2b478a9db4a13bdb

vmware_nsx/shell/admin/plugins/nsxv3/resources/ports.py

@@ -39,6 +39,7 @@ from neutron.db import db_base_plugin_v2
 from neutron.db import portsecurity_db
 from neutron.extensions import allowedaddresspairs
 from neutron_lib import constants as const
+from neutron_lib.plugins import directory
 
 LOG = logging.getLogger(__name__)
 
@@ -46,7 +47,12 @@ LOG = logging.getLogger(__name__)
 class PortsPlugin(db_base_plugin_v2.NeutronDbPluginV2,
                   portsecurity_db.PortSecurityDbMixin,
                   addr_pair_db.AllowedAddressPairsMixin):
-    pass
+    def __enter__(self):
+        directory.add_plugin(const.CORE, self)
+        return self
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        directory.add_plugin(const.CORE, None)
 
 
 def get_port_nsx_id(session, neutron_id):
@@ -109,8 +115,9 @@ def list_missing_ports(resource, event, trigger, **kwargs):
     """List neutron ports that are missing the NSX backend port
     And ports with wrong switch profiles
     """
-    plugin = PortsPlugin()
     admin_cxt = neutron_context.get_admin_context()
+
+    with PortsPlugin() as plugin:
         neutron_ports = plugin.get_ports(admin_cxt)
         port_client, profile_client = get_port_and_profile_clients()
 
@@ -217,9 +224,9 @@ def migrate_compute_ports_vms(resource, event, trigger, **kwargs):
         return
 
     # Go over all the compute ports from the plugin
-    plugin = PortsPlugin()
     admin_cxt = neutron_context.get_admin_context()
     port_filters = {'device_owner': ['compute:None']}
+    with PortsPlugin() as plugin:
         neutron_ports = plugin.get_ports(admin_cxt, filters=port_filters)
 
     for port in neutron_ports: