Fixes json conversion failure for custom values

What does this MR do and why?

This adds an enclosing double quotes for random username and passwords generated in prepare-custom-values go-task to avoid json conversion failure

How to set up and validate

1. Run go-task prepare-custom-values for a cluster and release. Notice custom generated username and password (e.g. clickhouse) enclosed in double quotes.
2. Try running helm install/helm upgrade for the new custom.value.yaml, it should not return any error.

Fixes https://gitlab.com/gitlab-org/analytics-section/product-analytics/analytics-stack/-/issues/163