SUBMARINE-643. Make git-sync image configurable

### What is this PR for?
For the known reason, this gcr image cannot be pulled from some places. We should make this configurable in helm charts and pass this to the submarine server by an environment variable.

In the other hand, we pushed this image to the Apache Docker hub with the name "apache/submarine:git-sync-3.1.6" as a default image.

`public static final String GIT_SYNC_IMAGE = "k8s.gcr.io/git-sync:v3.1.6";
`

### What type of PR is it?
[Improvement]

### Todos
* [ ] - Task

### What is the Jira issue?
https://issues.apache.org/jira/browse/SUBMARINE-643

### How should this be tested?
https://travis-ci.org/github/pingsutw/hadoop-submarine/builds/731185896

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No

Author: Kevin Su <pingsutw@gmail.com>

Closes #418 from pingsutw/SUBMARINE-643 and squashes the following commits:

c011b66 [Kevin Su] Make git-sync image configurable

submarine-server/server-submitter/submitter-k8s/src/main/java/org/apache/submarine/server/submitter/k8s/experiment/codelocalizer/GitCodeLocalizer.java

@@ -33,55 +33,55 @@
 
 public abstract class GitCodeLocalizer extends AbstractCodeLocalizer {
 
-  public static final String GIT_SYNC_IMAGE = "k8s.gcr.io/git-sync:v3.1.6";
-  
+  public static final String GIT_SYNC_IMAGE = "apache/submarine:git-sync-3.1.6";
+
   public GitCodeLocalizer(String url) {
     super(url);
   }
 
   public void localize(V1PodSpec podSpec) {
-    
+
     V1Container container = new V1Container();
     container.setName(CODE_LOCALIZER_INIT_CONTAINER_NAME);
     container.setImage(GIT_SYNC_IMAGE);
-    
+
     V1EnvVar repoEnv = new V1EnvVar();
     repoEnv.setName("GIT_SYNC_REPO");
     repoEnv.setValue(this.getUrl());
-    
+
     V1EnvVar rootEnv = new V1EnvVar();
     rootEnv.setName("GIT_SYNC_ROOT");
     rootEnv.setValue(CODE_LOCALIZER_PATH);
-    
+
     V1EnvVar destEnv = new V1EnvVar();
     destEnv.setName("GIT_SYNC_DEST");
     destEnv.setValue("current");
-    
+
     V1EnvVar oneTimeEnv = new V1EnvVar();
     oneTimeEnv.setName("GIT_SYNC_ONE_TIME");
     oneTimeEnv.setValue("true");
-    
+
     List<V1EnvVar> gitSyncEnvVars = new ArrayList<V1EnvVar>();
     gitSyncEnvVars.add(repoEnv);
     gitSyncEnvVars.add(rootEnv);
     gitSyncEnvVars.add(destEnv);
     gitSyncEnvVars.add(oneTimeEnv);
     container.setEnv(gitSyncEnvVars);
-    
+
     V1VolumeMount mount = new V1VolumeMount();
     mount.setName(CODE_LOCALIZER_MOUNT_NAME);
     mount.setMountPath(CODE_LOCALIZER_PATH);
-    
+
     List<V1VolumeMount> volumeMounts = new ArrayList<V1VolumeMount>();
     volumeMounts.add(mount);
-    
+
     container.setVolumeMounts(volumeMounts);
-    
+
     podSpec.addInitContainersItem(container);
-    
+
     super.localize(podSpec);
   }
-  
+
   public static CodeLocalizer getGitCodeLocalizer(String url)
       throws InvalidSpecException {