Variables inside secret files are not interpolated

**Describe the bug**
MLP does not interpolate variables when they are used inside a secret file.

E.g.
```json
{
"aKey": "{{VAR_TO_BE_INTERPOLATED}}"
}
```

**Expected behavior**
I expect that variables are correctly interpolated

**To Reproduce**
Steps to reproduce the behavior:
1. Add a secret file with interpolated variables in `mlp.yaml` of a Console project configuration.
2. Deploy
3. Get secret from k8s and notice that variables have not been interpolated

**Machine information (please complete the following information):**
- Version 1.1.0

Start with the secret-file configuration in mlp.yaml and trace how it is handled during deployment to Kubernetes. Reproduce the Console project configuration described in the issue, then verify the generated Kubernetes secret. Done means placeholders such as {{VAR_TO_BE_INTERPOLATED}} are replaced with their variable values.

Written by the indexing model from the issue text.