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.

Full upgrade scenario

This commit intoroduces more structured upgrade scenario consisting of
this scenarios:

• pre_upgrade_check - non-modifying checks: is the system ready?
• pre_migrations - modifying procedures before source update
• migrations - source update and main migrations...

Retain original objects while Tsorting. TSort create label graph and then convert to object graph

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...

Assumeyes option

Fixes #44 - Change tag 'basic' to 'default'

Better args handling

Allow running checks by label

Update readme

tests for health commands