Shellcheck fixes for deploy_to_kubernetes
Shellcheck fixes for deploy_to_kubernetes

- "SC2028: echo may not expand escape sequences. Use printf."
  https://github.com/koalaman/shellcheck/wiki/SC2038
- "SC2086: Double quote to prevent globbing and word splitting."
  https://github.com/koalaman/shellcheck/wiki/SC2086
  Also changed the "namespace" detection to use `test -z` instead and
  that means we can drop the `-u` flag in the shebang.
- "SC2002: Useless cat."
  https://www.shellcheck.net/wiki/SC2002
  Ignoring as I think it's clearer if `kubectl` commands are chained
  together using only stdin and stdout.

ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public,ministryofjustice/cla_public