[RHELC-601] Install subscription-manager using package manager

[r0x0d]

In this commit, we are introducing the installation of subscription-manager directly through a package manager, instead of downloading the package first and then installing it with system package manager.

This is necessary as we want to do GPG verification when installing those set of packages, in which case, it was not available when we used to download the packages and then install it via local install.

Notable changes in this commit are:

• Replacing installing subscription-manager to install it directly from the client-tools repository with gpg check enabled for all supported versions
• Minor refactor to RestorablePackageSet class to allow it to be generic and use custom repository paths and content to install packages
• Small tweak to call_yum_cmd to use the reposdir option as a list instead of a string
• Move subscription-manager constants to subscription.py module instead of the packages.py backup module

Jira Issues: RHELC-601

Checklist

• [ ] PR has been tested manually in a VM (either author or reviewer)
• [ ] Jira issue has been made public if possible
• [ ] [RHELC-] is part of the PR title
• [ ] GitHub label has been added to help with Release notes
• [ ] PR title explains the change from the user's point of view
• [ ] Code and tests are documented properly
• [ ] The commits are squashed to as few commits as possible (without losing data)
• [ ] When merged: Jira issue has been updated to Release Pending if relevant

---

Depends on

• [ ] https://github.com/oamg/convert2rhel/pull/1194