Switch init.d scripts to use bash

This commit modifies the init.d scripts to use bash now that bash is a
required dependency.

Relates #18308

distribution/deb/src/main/packaging/init.d/elasticsearch

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # /etc/init.d/elasticsearch -- startup script for Elasticsearch
 #

distribution/rpm/src/main/packaging/init.d/elasticsearch

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # elasticsearch <summary>
 #