Move kubernetes-entry.sh out of FLINK_HOME/bin

Currently, FLINK_HOME/bin contains the file kubernetes-entry.sh. This file is used to customize Flink's default Docker image. I think FLINK_HOME/bin should not contain files which cannot be directly used. Either we move them to another directory or we incorporate it into Flink's default Docker image. If we opt for the latter option, then this task is related to FLINK-12546.