In some places in the packaging tests we use heredocs to run some
scripts, for example, under the root user. However, we were running
these heredocs under sh instead of bash. This is confusing since we use
bash everywhere and we end up with syntax in the herdocs that expects
bash but will not necessarily be run under bash. For example, one
circumstance where this arises is the bash syntax [[. This leads to
packaging test failures on some systems (e.g., the debian-8 box) that
are solely due to this sh/bash distinction. We should really be using
bash everywhere, it is the shell that we settled on for all of our
scripts everywhere awhile ago and then we can avoid this confusion. This
commit replaces the usage of sh in the X-Pack packaging tests with bash.
Original commit: elastic/x-pack-elasticsearch@7555bb32c8
83aae92353