Comment 6 for bug 1402055

The issue here is that the Task Manager sets up an exists notification transformer during it's startup sequence:

(from taskmanager/manager.py)
     self.admin_context = TroveContext(
            user=CONF.nova_proxy_admin_user,
            auth_token=CONF.nova_proxy_admin_pass,
            tenant=CONF.nova_proxy_admin_tenant_id)
        if CONF.exists_notification_transformer:
            self.exists_transformer = importutils.import_object(
                CONF.exists_notification_transformer,
                context=self.admin_context)

The admin_context hasn't gone through the process of authenticating so the service_catalog property isn't set. During the initialization of the NovaNotificationTransformer:
(from extensions/mgmt/instances/models.py)
class NovaNotificationTransformer(NotificationTransformer):
    def __init__(self, **kwargs):
        super(NovaNotificationTransformer, self).__init__(**kwargs)
        self.context = kwargs['context']
        self.nova_client = remote.create_admin_nova_client(self.context)
        self._flavor_cache = {}

If you don't set CONF.nova_compute_url, the call to create_admin_nova_client will attempt to use the context.service_catalog which in this case is never set.

In order for this code to work, I believe the Task Manager is going to have to authenticate the admin_context with Keystone to get the service_catalog.

(BTW, the workaround mentioned above works because this code path is not executed if exists_notification_transformer is None.)