Stop unregistering the system by default

Satellite users are always registered through sub-man. And convert2rhel always re-registers the system, even if --keep-rhsm is passed. This poses a problem when the registration fails - Satellite loses the connection to the system.

Similarly for the Insights users. They may not only register the RHEL-based system to Insights using the insights-client but they can also run `rhc connect` which registers the system to both Insights and RHSM. The problem is that currently convert2rhel by default re-registers the system which causes the system to appear twice in the Insights Inventory. From rhn-engineering-cduryee:

if you do a re-register on rhsm, you'll get a new rhsm id. The rhsm ID is part of the insights canonical facts, which are the subset of facts used to fingerprint the system and check for duplicates (see https://github.com/RedHatInsights/insights-host-inventory/blob/master/app/serialization.py#L18-L28).

Also, UX-wise, it makes little sense to ask the user for RHSM credentials in order to perform the pre-conversion analysis or conversion when the system is already registered to RHSM.

Acceptance criteria

• Do not unregister the system through sub-man by default
• Unregister the system only when username/password or organization/activation key is provided by the user
• It may be needed to execute `subscription-manager refresh` for the sub-man to spot the RHEL RHSM product certificate that convert2rhel plants into the /etc/pki/product-default/