[proposal] Decouple Reservation plugin for the koord-scheduler

[saintube]

What is your proposal:

De-couple the Reservation plugin with the framework of the koord-scheduler. Make it available to use the koord-scheduler when the Resource Reservation ability is fully disabled.

Why is this needed:

Although Resource Reservation is a powerful mechanism, we should make it optional for the Koordinator. However, the current implementation uses the extension points of the reservation plugin coupled with the scheduling framework, e.g. ReservationNominator is always called at the end of the scheduling cycle to remove the nominated reserve pod, which means the koord-scheduler can not work well if the Reservation plugin is disabled or not registered. It makes it difficult for developers to know the Reservation before adopting the Koordinator's scheduler plugins.

Is there a suggested solution, if so, please add it:

[jueli12]

/assign