dynamic host volumes: node selection via constraints (#24518)

When making a request to create a dynamic host volumes, users can pass a node
pool and constraints instead of a specific node ID.

This changeset implements a node scheduling logic by instantiating a filter by
node pool and constraint checker borrowed from the scheduler package. Because
host volumes with the same name can't land on the same host, we don't need to
support `distinct_hosts`/`distinct_property`; this would be challenging anyways
without building out a much larger node iteration mechanism to keep track of
usage across multiple hosts.

Ref: https://github.com/hashicorp/nomad/pull/24479

command/agent/host_volume_endpoint_test.go

@@ -21,6 +21,8 @@ func TestHostVolumeEndpoint_CRUD(t *testing.T) {
 		// Create a volume on the test node
 
 		vol := mock.HostVolumeRequest(structs.DefaultNamespace)
+		vol.NodePool = ""
+		vol.Constraints = nil
 		reqBody := struct {
 			Volumes []*structs.HostVolume
 		}{Volumes: []*structs.HostVolume{vol}}

command/volume_create_host_test.go

@@ -11,13 +11,16 @@ import (
 	"github.com/hashicorp/hcl"
 	"github.com/hashicorp/nomad/api"
 	"github.com/hashicorp/nomad/ci"
+	"github.com/hashicorp/nomad/command/agent"
 	"github.com/mitchellh/cli"
 	"github.com/shoenig/test/must"
 )
 
 func TestHostVolumeCreateCommand_Run(t *testing.T) {
 	ci.Parallel(t)
-	srv, client, url := testServer(t, true, nil)
+	srv, client, url := testServer(t, true, func(c *agent.Config) {
+		c.Client.Meta = map[string]string{"rack": "foo"}
+	})
 	t.Cleanup(srv.Shutdown)
 
 	waitForNodes(t, client)
@@ -38,11 +41,6 @@ node_pool = "default"
 capacity_min = "10GiB"
 capacity_max = "20G"
 
-constraint {
-  attribute = "${attr.kernel.name}"
-  value     = "linux"
-}
-
 constraint {
   attribute = "${meta.rack}"
   value     = "foo"