Foreman Maintain

Add --whitelist option

Allows to mark some steps to be ignored in the results: allows to
continue even when some checks are failing.

Define order using ruby TSort

1. Introduce 2 metadata: 'before' and 'after'
2. Current order is:
Checks::SyncPlans::WithEnabledStatus
Checks::DiskSpeedMinimal
Checks::SystemRegistration
Checks::ForemanTasks::NotPaused
Checks::ForemanTasks::NotRunning
Checks::ForemanTasks::Invalid::CheckOld...

Support running commands interactively

RFE #23 - Add new status [WARNING] - Check: is system self registered

Fixes #3 - Add option to skip the prompted step