Use git-sync image from Docker hub

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 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";