Apply ShellCheck fixes to bash scripts

[frankgrimes97]

Fixes #662

Motivation

ShellCheck flagged a number of issues with the Bash scripts some of which could lead to unexpected behaviour.
e.g.

In scripts/pulsar/prepare_helm_release.sh line 21:
CHART_HOME=$(unset CDPATH && cd $(dirname "${BASH_SOURCE[0]}")/../.. && pwd)
                                ^----------------------------^ SC2046 (warning): Quote this to prevent word splitting.

Modifications

I've attempted to address most of the issues reported by ShellCheck 0.11.0.
For other seeming false positives I suppressed them using ShellCheck ignore directives

e.g. https://www.shellcheck.net/wiki/SC2016

$ grep shellcheck -A1 .ci/helm.sh 
# shellcheck disable=SC2034
KIND_BIN=$OUTPUT_BIN/kind
--
      # shellcheck disable=SC2016
      timeout 120s "${KUBECTL}" exec -n "${NAMESPACE}" "${CLUSTER}"-toolset-0 -- bash -c 'until [ "$(curl -s -L http://pulsar-ci-broker:8080/status.html)" == "OK" ]; do sleep 3; done' || { echo >&2 "Timeout waiting..."; ci::print_pod_logs; exit 1; }

Verifying this change

I'm not sure how exhaustively these scripts are covered by CI tests and haven't manually tested all of them due to my lack of familiarity with the project so some guidance/assistance may be required.

[frankgrimes97]

I see the CI checks show a lot of red... I'll try to dig in and see how my changes might have broken things. 😄

[frankgrimes97]

CI is back to green now so I think this should be ready for review.