move spool dir to make it fully configurable per ejabberdctl.cfg (#3863)

In the packaged rpm the spool dir is set to:
```
: "${SPOOL_DIR:="/opt/ejabberd/database/$ERLANG_NODE"}"
```

However, `$ERLANG_NODE` is effectively set later (now in line 67), which effectively makes spool dir always in `...../ejabberd@localhost`

Author: duritong

ejabberdctl.template

@@ -59,7 +59,6 @@ done
 # define ejabberd variables if not already defined from the command line
 : "${CONFIG_DIR:="{{config_dir}}"}"
 : "${LOGS_DIR:="{{logs_dir}}"}"
-: "${SPOOL_DIR:="{{spool_dir}}"}"
 : "${EJABBERD_CONFIG_PATH:="$CONFIG_DIR/ejabberd.yml"}"
 : "${EJABBERDCTL_CONFIG_PATH:="$CONFIG_DIR/ejabberdctl.cfg"}"
 # Allows passing extra Erlang command-line arguments in vm.args file
@@ -68,6 +67,7 @@ done
 [ -f "$EJABBERDCTL_CONFIG_PATH" ] && . "$EJABBERDCTL_CONFIG_PATH"
 [ -n "$ERLANG_NODE_ARG" ] && ERLANG_NODE="$ERLANG_NODE_ARG"
 [ "$ERLANG_NODE" = "${ERLANG_NODE%.*}" ] && S="-s"
+: "${SPOOL_DIR:="{{spool_dir}}"}"
 : "${EJABBERD_LOG_PATH:="$LOGS_DIR/ejabberd.log"}"
 
 # define erl parameters