Add script to run shellcheck on bash scripts in the repository

[sayantani11]

/kind feature

Describe the solution you'd like
A lot of CI jobs depend on our bash scripts, we should add shellcheck as part of presubmit to avoid small mistakes.

• Add a bash script to run shellcheck on all the scripts in the repository. Derived from Kubernetes Shellcheck
• Add verify-shellcheck make target and include it as a part of make verify.
• Fix shellcheck warnings in the repository.

[sayantani11]

/assign