fix merged env vars, new vars should overwrite the old one (#595)

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# Description
<!-- Briefly describe the motivation for the change. Please include
illustrations where appropriate. -->
When user want to deploy and add the environment variables of Merlin
model's endpoint, instead of overwriting the old environment variables
with the new variables, both of the variables will be [appended
instead](https://github.com/caraml-dev/merlin/blob/5b37909b8ee3972469f2fd2a4677596af78c7f99/api/service/version_endpoint_service.go#L235).
E.g.

- Current endpoint's vars: `A = false`
- New endpoint's vars: `B = true`
- Final deployed endpoint's vars: `A = false, B = true`

These changes are intended to fix it, so when user updates the endpoint
environment variables, it will overwrite the old environment variables.

# Modifications
<!-- Summarize the key code changes. -->

Modify the `override` function, which is use to override left endpoint
with right endpoint, so it will replace the environment variables of the
left (old) version endpoint with right (new) version endpoint's
environment variables.

# Tests
<!-- Besides the existing / updated automated tests, what specific
scenarios should be tested? Consider the backward compatibility of the
changes, whether corner cases are covered, etc. Please describe the
tests and check the ones that have been completed. Eg:
- [x] Deploying new and existing standard models
- [ ] Deploying PyFunc models
-->

# Checklist
- [x] Added PR label
- [x] Added unit test, integration, and/or e2e tests
- [x] Tested locally
- [ ] Updated documentation
- [ ] Update Swagger spec if the PR introduce API changes
- [ ] Regenerated Golang and Python client if the PR introduces API
changes

# Release Notes
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```release-note
deploying or redeploying model's endpoint with environment variables will now replace all old environment variables with new ones
```

Co-authored-by: bthari.smartnastiti <[email protected]>

api/service/version_endpoint_service.go

@@ -231,8 +231,8 @@ func (k *endpointService) override(left *models.VersionEndpoint, right *models.V
 
 	// override env vars
 	// Configure environment variables for Pyfunc model
-	if len(right.EnvVars) > 0 {
-		left.EnvVars = models.MergeEnvVars(left.EnvVars, right.EnvVars)
+	if right.EnvVars != nil {
+		left.EnvVars = right.EnvVars
 	}
 
 	// override protocol

api/service/version_endpoint_service_test.go

@@ -669,6 +669,12 @@ func TestDeployEndpoint(t *testing.T) {
 								CPURequest:    resource.MustParse("1"),
 								MemoryRequest: resource.MustParse("1Gi"),
 							},
+							EnvVars: models.EnvVars{
+								{
+									Name:  "TF_MODEL_NAME",
+									Value: "saved_model.pb",
+								},
+							},
 							Logger: &models.Logger{
 								Model: &models.LoggerConfig{
 									Enabled: true,
@@ -697,6 +703,12 @@ func TestDeployEndpoint(t *testing.T) {
 						CPURequest:    resource.MustParse("1"),
 						MemoryRequest: resource.MustParse("1Gi"),
 					},
+					EnvVars: models.EnvVars{
+						{
+							Name:  "NUM_OF_ITERATION",
+							Value: "1",
+						},
+					},
 					Logger: &models.Logger{
 						Model: &models.LoggerConfig{
 							Enabled: true,
@@ -723,6 +735,12 @@ func TestDeployEndpoint(t *testing.T) {
 					CPURequest:    resource.MustParse("1"),
 					MemoryRequest: resource.MustParse("1Gi"),
 				},
+				EnvVars: models.EnvVars{
+					{
+						Name:  "NUM_OF_ITERATION",
+						Value: "1",
+					},
+				},
 				Logger: &models.Logger{
 					DestinationURL: loggerDestinationURL,
 					Model: &models.LoggerConfig{