Foreman Maintain

Cloned from https://bugzilla.redhat.com/show_bug.cgi?id=1544769

Description of problem:
From satellite v 6.3 onwards, we are not supporting self-registered satellite server, reference [1] and [2]

The foreman-maintain should add this check and notify the users accordingly in pre-upgrade check and ask customer follow upgrade steps for disconnected satellite server.

It will be nice if we could point the KCS / Doc link for the disconnected upgrade.

Version-Release number of selected component (if applicable):
Red Hat Satellite 6.3 Upgrade

[1] https://access.redhat.com/documentation/en-us/red_hat_satellite/6.3-beta/html/installation_guide/preparing_your_environment_for_installation#system_requirements

[2] https://access.redhat.com/documentation/en-us/red_hat_satellite/6.3-beta/html/upgrading_and_updating_red_hat_satellite/upgrading_red_hat_satellite#migrating_a_self_registered_satellite