fix(chart): default values now use an existing secret to fix deployment

[gCyrille]

What? (description)

This PR set a default value for existingConfigSecret to match the installation documentation (section Method 1: kubectl)

Fixes #74

Why? (reasoning)

After the previous changes to Allow usage of env var instead of secret config file for xo config, the volume mount for the secret was removed from the deployments.

Acceptance

Please use the following checklist:

• you linked an issue (if applicable)
• you included tests (if applicable)
• you linted your code (make lint)
• you linted your code (make unit)

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[JeremyLARDENOIS]

my bad, i didn't look about existingConfigSecret 😅