Update the `populate_switch_ports` background task for multi-rack

The populate_switch_ports task in Nexus implements a necessary part of the rack setup process. It looks up the dpd clients for the local switches, gets lists of ports from them, and populates the switch_port database table accordingly. The current version of the task uses the local configured rack ID for everything. For multirack, we'll either need to restrict the lookup so that we only query the dpd instances on the local rack, or we'll need to make other changes to the task if we want Nexus to be able to run this task for another rack's switches.

Likely depends on #5201 so that we can specifically find the local rack's instances of dpd.

Locate the populate_switch_ports background task and review issue #5201 first, since the issue says it may provide local-rack instance lookup. Determine how the task should select DPD clients for multirack, then verify that the appropriate ports populate the switch_port table for the target rack.