Add priority serializers for different modes

The commit introduce a new being - priority serializer. We need such
being since we should use different priority strategies for different
cases.

For example:

* We should deploy secondary controllers one-by-one in 5.0.x clusters.
* We should deploy all nodes one-by-one in case of patching.

Closes-Bug: #1364532
Change-Id: I77a445a51576b59aa188128047780af4db804312