Strange behaviour when use an existing secret and databbase root password

[jrpedrianes]

If we use an existing secret to store credentials, the modifications included in #43 forces us to add a dummy database root password.

This behaviour is a bit strange, maybe would be better to add a new helm value to force the environment mapping, or change the way the secrets are included as environment variables, ie, map a secret as environment variables directly: https://kubernetes.io/docs/concepts/configuration/secret/#use-cases

[drpebcak]

I've opened a PR to address this. Please take a look if you get a chance.

[jrpedrianes]

LGTM, now I think it is more consistent to do it in that way

[drpebcak]

Closing as #49 is now merged