Make admin client more configurable

[jclementemis]

As user
I want to configure admin client via environment variables
So that I can change remoras behaviour at runtime

Description

Currently I believe that it is not possible to add additional properties to admin client via environment variables.
https://github.com/zalando-incubator/remora/blob/master/src/main/resources/application.conf#L24

How I would like to use this feature ?

• I run remora in a kubernetes environment and I want to connect remora to AWS MSK instance that has SSL enabled between client and broker. I would like to supply SSL security.protocol to the admin client via kubernetes environment variable or configMaps.

Acceptance criteria

• A user should be able to configure properties of the admin-client e.g security.protocol via environment variables

[soceanainn]

There are a lot of potential configuration options for configuring the admin-client properties, and it would be a lot of work to maintain the ability to set any of them as environment variables (which also has additional challenges when handling nested structured date). We added the ability to configure admin-client properties specifically for the use case you are describing but made the conscious decision not to support doing this using env variables.

You can overwrite the configuration file by storing an inline conf file in a Kubernetes configmap, and then mounting that as a file to your pods.