Consider changing MaxClusters plugin to respect avoidMigration option

[limhawjia]

The MaxClusters scheduler plugin does not have any preference for current placements. This undermines the avoidMigration field in propagation policies when maxClusters is set. We can consider adding support for the avoidMigration field.

The new behavior when avoidDisruption == true would look something like:

if len(currentClusters) == maxClusters {
    return currentClusters
} else if len(currentClusters) > maxClusters {
    // remove lowest scoring clusters
    removeLowestScoreClusters(currentClusters, clusterScores, maxClusters)
    return currentClusters
} else {
    // add highest scoring clusters that are not already present
    addHighestScoreClusters(currentClusters, clusterScores, maxClusters)
    return currentClusters
}