feat: Experimental Helm v3 mode (#841)

Set `HELMFILE_HELM3=1` and run `helmfile` like `HELMFILE_HELM3=1 helmfile ...`.

When `HELMFILE_HELM3` is set, `test`, `template`, `delete`, `destroy` behave differently so that it works with Helm 3.

Note that `helmfile diff` doesn't work as `helm-diff` called under the hood doesn't support Helm v3 yet.

Ref #668

pkg/state/state.go

@@ -896,7 +896,7 @@ func (st *HelmState) DeleteReleases(affectedReleases *AffectedReleases, helm hel
 		}
 
 		flags := []string{}
-		if purge {
+		if purge && !st.isHelm3() {
 			flags = append(flags, "--purge")
 		}
 		flags = st.appendConnectionFlags(flags, &release)
@@ -930,13 +930,21 @@ func (st *HelmState) TestReleases(helm helmexec.Interface, cleanup bool, timeout
 		if cleanup {
 			flags = append(flags, "--cleanup")
 		}
-		flags = append(flags, "--timeout", strconv.Itoa(timeout))
+		duration := strconv.Itoa(timeout)
+		if st.isHelm3() {
+			duration += "s"
+		}
+		flags = append(flags, "--timeout", duration)
 		flags = st.appendConnectionFlags(flags, &release)
 
 		return helm.TestRelease(st.createHelmContext(&release, workerIndex), release.Name, flags...)
 	})
 }
 
+func (st *HelmState) isHelm3() bool {
+	return os.Getenv("HELMFILE_HELM3") != ""
+}
+
 // Clean will remove any generated secrets
 func (st *HelmState) Clean() []error {
 	errs := []error{}
@@ -1250,9 +1258,7 @@ func (st *HelmState) flagsForUpgrade(helm helmexec.Interface, release *ReleaseSp
 }
 
 func (st *HelmState) flagsForTemplate(helm helmexec.Interface, release *ReleaseSpec, workerIndex int) ([]string, error) {
-	flags := []string{
-		"--name", release.Name,
-	}
+	flags := []string{}
 
 	var err error
 	flags, err = st.appendHelmXFlags(flags, release)