clarify elasticsearch script as bash

[BenTheElder]

What this PR does / why we need it: eliminates shellcheck failure in cluster/addons/fluentd-elasticsearch/es-image/run.sh by correcting the shebang to bash.

NOTE:

• The base image / existing published image are centos based with bash. /bin/sh is bash 4.2. This does not change what shell it actually runs under.
• $HOSTNAME is not POSIX, however it is set by bash.

Which issue(s) this PR fixes:

part of #72956

Special notes for your reviewer:

$ docker run --rm --entrypoint=/bin/sh gcr.io/fluentd-elasticsearch/elasticsearch:v6.7.2 --version
GNU bash, version 4.2.46(2)-release (x86_64-redhat-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Does this PR introduce a user-facing change?:

NONE

/kind cleanup
/sig cluster-lifecycle
/priority important-longterm

[BenTheElder]

/assign @spiffxp

[BenTheElder]

commit history suggests this was set to /bin/sh in order to "rebase onto alpine", however that's not actually what the image is FROM today 🤷‍♂

$ docker run --rm --entrypoint=/bin/sh gcr.io/fluentd-elasticsearch/elasticsearch:v6.7.2 -c 'cat /etc/os-release'
NAME="CentOS Linux"
VERSION="7 (Core)"
ID="centos"
ID_LIKE="rhel fedora"
VERSION_ID="7"
PRETTY_NAME="CentOS Linux 7 (Core)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:centos:centos:7"
HOME_URL="https://www.centos.org/"
BUG_REPORT_URL="https://bugs.centos.org/"

CENTOS_MANTISBT_PROJECT="CentOS-7"
CENTOS_MANTISBT_PROJECT_VERSION="7"
REDHAT_SUPPORT_PRODUCT="centos"
REDHAT_SUPPORT_PRODUCT_VERSION="7"

[BenTheElder]

/remove-sig cluster-lifecycle

[fejta]

/lgtm
/approve

[BenTheElder]

/assign @spiffxp

[fejta]

/lgtm
/approve

[spiffxp]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: BenTheElder, fejta, spiffxp

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• cluster/addons/fluentd-elasticsearch/OWNERS [spiffxp]
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