Composable filters for node scheduling

[coryodaniel]

Add submodules / functions to Bonny.Server.Scheduler for composable filtering of nodes:

• affinity / anti-affinity for pods / nodes
• labels
• taints

Blocked by #55

subset_of_nodes = 
  Bonny.Server.Scheduler.Nodes.with_labels(["label-key", {"a-key", "with-a-value"}])
  |> Bonny.Server.Scheduler.Nodes.with_affinity(...)
  |> Bonny.Server.Scheduler.nodes()