Do not always init compute_client when doint port list

This patch ensures that compute client is initialized only when needed
(--server arg is passed) to openstack port list command. Otherwise it
leads to failures on installations without Nova.

Change-Id: I102683461daa2f7d05dd9d7a3ec72de551c65ca9
Closes-Bug: #1688194

openstackclient/network/v2/port.py

@@ -519,7 +519,6 @@ def get_parser(self, prog_name):
 
     def take_action(self, parsed_args):
         network_client = self.app.client_manager.network
-        compute_client = self.app.client_manager.compute
         identity_client = self.app.client_manager.identity
 
         columns = (
@@ -548,6 +547,7 @@ def take_action(self, parsed_args):
                                                  ignore_missing=False)
             filters['device_id'] = _router.id
         if parsed_args.server:
+            compute_client = self.app.client_manager.compute
             server = utils.find_resource(compute_client.servers,
                                          parsed_args.server)
             filters['device_id'] = server.id

releasenotes/notes/bug-1688194-bb008b65267a1169.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fix issue in ``port list`` command when no Compute endpoint is in the
+    Service Catalog.
+    [Bug `1688194 <https://bugs.launchpad.net/bugs/1688194>`_]