fix(#507): support assign --post-renderer within helmfile flags and helmdefault or release config

1. only implement post-renderer flags this patch
2. As mumoshu advise, add helmfile flags `--post-render` and add the
   postRenderer  config in helmDefaults and release. the priority is
   helmfile flags > release > helmDefaults.
3. fix the test case in state_test.go and some other tests.

Signed-off-by: guofutan <guofutan@tencent.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

pkg/helmexec/helmexec.go

@@ -14,6 +14,8 @@ type Interface interface {
 	SetExtraArgs(args ...string)
 	SetHelmBinary(bin string)
 	SetEnableLiveOutput(enableLiveOutput bool)
+	SetPostRenderer(postRenderer string)
+	GetPostRenderer() string
 
 	AddRepo(name, repository, cafile, certfile, keyfile, username, password string, managed string, passCredentials string, skipTLSVerify string) error
 	UpdateRepo() error