Helm: existingSecret support for redis and mysql

[timchenko-a]

Right now both external MySQL and external Redis support only explicit password setup, while some services, e.g. RabbitMQ and PostgreSQL already support using existing secret.
Main request of this issue is to add existingSecret support to Redis and MySQL. Otherwise this causes security issues when managing Helm charts in a declarative way, e.g. with helm controller.
I can work on this, unless somebody else has already started.

[Matvey-Kuk]

@timchenko-a I'm not aware of anybody working on this feature, would be happy to see your contribution!

[timchenko-a]

@Matvey-Kuk there is a PR resolving this issue (#1016). Would you be so kind as to help with getting some review there?

[drehelis]

Done in #2156