Specify max nodes instead of CBF in placement policy

Is your feature request related to a problem? Please describe.

Selectors use CBF to have some flexibility over the node count used for container. For REP containers it generally worked fine, with a typical "REP 3" policy we could have 3-6-9-12-etc max nodes used with the minimum of 3 (typically, depends on selector of course). However EC containers require more nodes in general, real policies start with 4/2 and end up somewhere in 12/4 range. Which means that CBF applied there makes the node count grow from 16 right into 32 for the first step. Which works, but can be suboptimal for containers used via S3 (SEARCH).

Describe the solution you'd like

Have some way to specify the maximum number of nodes used without CBF, like "NODES 20" to use from whatever minimum number of nodes to 20.

Describe alternatives you've considered

Keep things as is?

Additional context

Related to #383.

No file or test is named. Start by locating the placement policy definition and reviewing related issue #383 to understand the existing CBF behavior. Define how an NODES limit applies from the policy's minimum node count, then verify that the resulting policy representation and documentation express the requested maximum clearly.