kola: use platform.Conf instead of an Ignition spec directly

The `platform.Conf` type allows abstracting over the different Ignition
versions so that different tests can use different versions. Using it
instead of the Ignition type directly means that we can now use the
3.2-experimental spec in external tests.

This is needed for testing the new LUKS support in Ignition[1] and the
related rootfs-on-complex-devices work[2].

[1] coreos/ignition#960
[2] coreos/fedora-coreos-config#503

Closes: coreos#1589

mantle/kola/harness.go

@@ -31,8 +31,6 @@ import (
 	"github.com/kballard/go-shellquote"
 	"github.com/pkg/errors"
 
-	ignv3 "github.com/coreos/ignition/v2/config/v3_0"
-	ignv3types "github.com/coreos/ignition/v2/config/v3_0/types"
 	"github.com/coreos/mantle/harness"
 	"github.com/coreos/mantle/harness/reporters"
 	"github.com/coreos/mantle/kola/cluster"
@@ -57,7 +55,6 @@ import (
 	"github.com/coreos/mantle/platform/machine/unprivqemu"
 	"github.com/coreos/mantle/sdk"
 	"github.com/coreos/mantle/system"
-	"github.com/coreos/mantle/util"
 )
 
 // InstalledTestsDir is a directory where "installed" external
@@ -603,7 +600,7 @@ func runExternalTest(c cluster.TestCluster, mach platform.Machine) error {
 }
 
 func registerExternalTest(testname, executable, dependencydir, ignition string, baseMeta externalTestMeta) error {
-	ignc3, _, err := ignv3.Parse([]byte(ignition))
+	config, err := conf.Ignition(ignition).Render("", Options.IgnitionVersion == "v2")
 	if err != nil {
 		return errors.Wrapf(err, "Parsing config.ign")
 	}
@@ -627,26 +624,7 @@ Environment=KOLA_UNIT=%s
 Environment=%s=%s
 ExecStart=%s
 `, KoletExtTestUnit, kolaExtBinDataEnv, kolaExtBinDataDir, remotepath)
-	runextconfig := ignv3types.Config{
-		Ignition: ignv3types.Ignition{
-			Version: "3.0.0",
-		},
-		Systemd: ignv3types.Systemd{
-			Units: []ignv3types.Unit{
-				{
-					Name:     KoletExtTestUnit,
-					Contents: &unit,
-					Enabled:  util.BoolToPtr(false),
-				},
-			},
-		},
-	}
-
-	finalIgn := ignv3.Merge(ignc3, runextconfig)
-	serializedIgn, err := json.Marshal(finalIgn)
-	if err != nil {
-		return errors.Wrapf(err, "serializing ignition")
-	}
+	config.AddSystemdUnit(KoletExtTestUnit, unit, conf.Enable)
 
 	t := &register.Test{
 		Name:          testname,
@@ -677,7 +655,7 @@ ExecStart=%s
 			}
 		},
 
-		UserDataV3: conf.Ignition(string(serializedIgn)),
+		UserDataV3: conf.Ignition(config.String()),
 	}
 
 	// To avoid doubling the duplication here with register.Test, we support