[CentOS8]tripleo modify images fail when gating-repo is not present

[yatin]

When gating-repo or any other repo defined in update_repo is not present tripleo-modify-image fails with stderr: 'Error: Unknown repo: ''gating-repo'''

Example log:-
https://260e67f8ff0e2abb7e19-6ad58f996ee59ccfe54ab476a81112a3.ssl.cf2.rackcdn.com/710091/4/check/tripleo-ci-centos-8-scenario002-standalone/e13c262/logs/undercloud/home/zuul/ansible.log

In CentOS7 yum used to ignore unavailable repos, but with CentOS8/Rhel8 dnf fails.

Need to handle unavailable repos to fix issue in CentOS8.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/711627

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/711627
Committed: https://git.openstack.org/cgit/openstack/tripleo-quickstart-extras/commit/?id=d70ee0de924d7b6536c190a42450fa235ab28cd2
Submitter: Zuul
Branch: master

commit d70ee0de924d7b6536c190a42450fa235ab28cd2
Author: yatinkarel <email address hidden>
Date: Fri Mar 6 19:45:29 2020 +0530

    Add gating repo to update repos only if it's setup

    gating repo is setup only when packages get's build,
    so add gating repo to update repos only when gating
    repo is setup i.e based on compressed_gating_repo.

    Also add new role variable container_prep_additional_repos,
    which can be used to add more repos for container updates.

    Closes-Bug: #1865459
    Change-Id: I9e7b2e04182d156502c1fdb12bcb743d800a6b49