Config: Unset the default config value

This PR make sure that if user try to set a default value for a
config property then it just unset that property and default value
is used. It will also allow not to have default value in the viper
config file. Also add `UpdateDefaults` to set and get config handler
so API can also make use of it.

Without this Patch
```
$ crc config set preset microshift
$ crc config set cpus 2
$ crc config set preset openshift
$ cat ~/.crc/crc.json
{
  "cpus": 2,
  "preset": "openshift"
}
```

After this patch `~/.crc/crc.json` going to be empty and openshift
preset is going to use default `cpus` as `4`.

pkg/crc/api/handlers.go

@@ -191,6 +191,7 @@ func (h *Handler) SetConfig(c *context) error {
 	if len(multiError.Errors) != 0 {
 		return multiError
 	}
+	crcConfig.UpdateDefaults(h.Config)
 	return c.JSON(http.StatusOK, client.SetOrUnsetConfigResult{
 		Properties: successProps,
 	})
@@ -219,6 +220,7 @@ func (h *Handler) UnsetConfig(c *context) error {
 	if len(multiError.Errors) != 0 {
 		return multiError
 	}
+	crcConfig.UpdateDefaults(h.Config)
 	return c.JSON(http.StatusOK, client.SetOrUnsetConfigResult{
 		Properties: successProps,
 	})

pkg/crc/config/config.go

@@ -95,6 +95,17 @@ func (c *Config) Set(key string, value interface{}) (string, error) {
 		return "", fmt.Errorf(invalidType, value, key)
 	}
 
+	// Make sure if user try to set same value which
+	// is default then just unset the value which
+	// anyway make it default and don't update it
+	// ~/.crc/crc.json (viper config) file.
+	if setting.defaultValue == castValue {
+		if _, err := c.Unset(key); err != nil {
+			return "", err
+		}
+		return "", nil
+	}
+
 	if setting.isSecret {
 		if err := c.secretStorage.Set(key, castValue); err != nil {
 			return "", err

pkg/crc/config/viper_config_test.go

@@ -115,13 +115,14 @@ func TestViperConfigLoadDefaultValue(t *testing.T) {
 		Value:     4,
 		IsDefault: true,
 	}, config.Get(cpus))
-
 	_, err = config.Set(cpus, 4)
 	assert.NoError(t, err)
 
 	bin, err := os.ReadFile(configFile)
 	assert.NoError(t, err)
-	assert.JSONEq(t, `{"cpus":4}`, string(bin))
+	// Setting default value will not update
+	// write to the config so expected would be {}
+	assert.JSONEq(t, `{}`, string(bin))
 
 	assert.Equal(t, SettingValue{
 		Value:     4,