Comment 6 for bug 1785597

Reviewed: https://review.openstack.org/604165
Committed: https://git.openstack.org/cgit/openstack/tripleo-common/commit/?id=01a4d265e30a0a0f03db25e5955da4d1b7b27a82
Submitter: Zuul
Branch: stable/queens

commit 01a4d265e30a0a0f03db25e5955da4d1b7b27a82
Author: Jose Luis Franco Arza <email address hidden>
Date: Mon Aug 6 15:28:22 2018 +0200

    Avoid getting one-empty-element-list in blacklisted_hostnames.

    When obtaining the output from the heat.stack_output_show
    action [0], if no parameter is specified in the stack, then
    the action returns a list of one empty string ['']. This
    is causing some error when calling tripleo.ansible-playbook
    workbook, as self.blacklisted_hostnames get set, as the
    argument passed is a non-empty list [1], and the result is
    that the argument '!' is always passed to the --limit
    option when calling ansible-playbook.

    [0] - https://github.com/openstack/tripleo-common/blob/master/workbooks/deployment.yaml#L365
    [1] - https://github.com/openstack/tripleo-common/blob/master/tripleo_common/actions/ansible.py#L461-L464

    Change-Id: I559d5ec5548ca24490a204809c7167660c307bac
    Closes-Bug: #1785597
    (cherry picked from commit 1ad01c3335b3f788b8c84f49ab82cf508c0b65c0)