Bookkeeper docker script does not support passing multiple ledger&journal files

**BUG REPORT**

***Describe the bug***

The config [here](https://github.com/apache/bookkeeper/blob/master/conf/bk_server.conf#L368) says Bookkeeper supports multiple ledger files. However, it won't work properly if we specify something like `BK_ledgerDirectories=/tmp/bk-ledgers1,/tmp/bk-ledgers2` as the script [here](https://github.com/apache/bookkeeper/blob/master/docker/scripts/common.sh#L79) seems not supporting such format. Also, [here](https://github.com/apache/bookkeeper/blob/master/conf/bk_server.conf#L279) says it is preferable to use `journalDirectories`, but script above is still using `journalDirectory`.

***Expected behavior***

The docker script should support passing multiple ledger&journal files.