Add option to skip the update/reboot on pre-upgraded server

[rjo-uk]

As part of the upgrade role, an initial update and reboot is performed in https://github.com/redhat-cop/infra.leapp/blob/main/roles/upgrade/tasks/leapp-upgrade.yml

Whilst this is good practice, it would be useful to have the option of disabling this so that:

• Any updates that are available on the server are not automatically applied. For example, updates from a third party repository may bring in unwanted packages (eg a new third party kernel) and result in a server that cannot be upgraded
• Later code in the playbook may fail gracefully (for example, due lack of disk space) before the upgrade is performed. If no additional updates have been installed on the current operating system, it is easier to recover the server to the pre-upgrade state.
• The pre-upgrade update can be performed during a different change window ahead of upgrade work. This will reduce upgrade time by not having to perform a reboot.

This initial draft is intended to not change workflows for existing users. If the variable skip_pre_upgrade_update_reboot is not provided, the update and reboot will occur as it does today.