Document how to use pulp_installer in disconnected mode

[pulpbot]

Author: @mikedep333 (mdepaulo@redhat.com)

Redmine Issue: 9087, https://pulp.plan.io/issues/9087

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Once we refactor the repos logic into the pulp_repos role, we should document how to use pulp_installer in disconnected mode with RPMs.

We have had 2 users requests for this recently.

My IRC explanation:

    * For a disconnected install, you need to use RPMs (install from a yum a mirror of pulp) rather than installing from pip packages, and you need to add offline copies of repos to the system yourself beforehand.
    * https://pulp-installer.readthedocs.io/en/latest/roles/pulp_common/#role-variables-if-installing-from-rpms
        * set pulp_install_plugins to packages
        * Set pulp_pkg_repo to your network's copy of https://yum.theforeman.org/pulpcore/3.7/el{{ ansible_distribution_major_version }}/$basearch/
        * Add the other repos like the centos software collection (that it tried to add) manually before running the installer.
        * You also need to modify the files under /etc/yum.repos.d/ that got added by the packages centos-release-scl-rh and centos-release-scl so that they are disabled, and your network's mirror is used instead.
    * I've been meaning to implement a better alternative to #4: https://pulp.plan.io/issues/8701
    * Also, this requires CentOS 7 / RHEL 7 or CentOS 8 / RHEL 8. Because RPM packages only exist for them (and they are actually provided by our sister project Foreman, and they do not make every plugin available.)

Also, we are not going to try to make pulp_devel work offline

[stale]

This issue has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!

[lubosmj]

We do not support pulp-installer anymore.