feat(mozcloud): externalSecrets -> secrets in container defs, syntax fixes

[emaydeck-mozilla]

One bigger change and one smaller change:

• Bigger change: container and initContainer definitions now allow you to specify secrets under secrets instead of externalSecrets.
  • This was already true previously, but has since been renamed and reworded to make things clearer and more accurate.
  • The readme has been updated to clarify what goes where.
  • New test cases were added to the external secrets test to ensure that we can mount secrets created outside this chart.
• Smaller change: Removed unnecessary parentheses for some default statements. These still render the same way.