[OVS] "vsctl" implementation does not allow empty transactions

[Rodolfo Alonso]

"vsctl" implementation was removed in Train. [1] introduced in Stein a bug when using the "vsctl" implementation. A transaction using this implementation cannot be empty, should have at least one operation.

In [2], if qos and queue are None, this transaction won't have any operation. As commented, in "vsctl" implementation this will raise the following exception:
  Exit code: 1; Stdin: ; Stdout: ; Stderr: ovs-vsctl: missing command name (use --help for help)

**Only stable Stein is affected with this problem.**

[1]https://review.opendev.org/#/c/738171/
[2]https://review.opendev.org/#/c/738171/1/neutron/agent/common/ovs_lib.py@914

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/stein
Review: https://review.opendev.org/738574

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/738574
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=ca11545e06fbe5f8ad0467c99c84d6a5bacbb895
Submitter: Zuul
Branch: stable/stein

commit ca11545e06fbe5f8ad0467c99c84d6a5bacbb895
Author: Rodolfo Alonso Hernandez <email address hidden>
Date: Tue Jun 30 09:56:29 2020 +0000

    Avoid empty "vsctl" transactions

    The OVS DB "vsctl" implementation does not allow empty transactions.
    In [1], if qos and queue are None, this transaction won't have any
    operation. In "vsctl" implementation this will raise the following
    exception:
      Exit code: 1; Stdin: ; Stdout: ; Stderr: ovs-vsctl: missing command
        name (use --help for help)

    [1]https://review.opendev.org/#/c/738171/1/neutron/agent/common/ovs_lib.py@914

    Change-Id: Iece42986b32e774018affbbbe83923ce73590d77
    Closes-Bug: #1885695