Properly handle not existing secrets

[hirenko-v]

It would be great to have some "fallcack" configuration for placeholders.

In our case application consists of multiple resources one of them creates secrets which application should use when it will appear.
If secret is not existing we'd like to sert some fallback value to placeholder and do not fail with the following error

failed exit status 1: Error: Replace: could not replace all placeholders in Template: