skip eviction when pod creation time is below minPodAge threshold setting (#1475)

* skip eviction when pod creation time is below minPodAge threshold setting

In the default initialization phase of the descheduler, add a new
constraint to not evict pods that creation time is below minPodAge
threshold.

Added value:

- Avoid crazy pod movement when the autoscaler scales up and down.

- Avoid evicting pods when they are warming up.

- Decreases the overall cost of eviction as no pod will be evicted
  before doing significant amount of work.

- Guard against scheduling. Descheduling loops in situations where
  the descheduler has a different node fit logic from scheduler,
  like not considering topology spread constraints.

* Use *time.Duration instead of uint for MinPodAge type

* Remove '(in minutes)' from default evictor configuration table

* make fmt

* Add explicit name for Duration field

* Use Duration.String()

README.md

@@ -145,6 +145,7 @@ The Default Evictor Plugin is used by default for filtering pods before processi
 |`priorityThreshold`|`priorityThreshold`||(see [priority filtering](#priority-filtering))|
 |`nodeFit`|`bool`|`false`|(see [node fit filtering](#node-fit-filtering))|
 |`minReplicas`|`uint`|`0`| ignore eviction of pods where owner (e.g. `ReplicaSet`) replicas is below this threshold |
+|`minPodAge`|`metav1.Duration`|`0`| ignore eviction of pods with a creation time within this threshold |
 
 ### Example policy