Replace specific "Nexus" mentions with more generic "artifact repository"

[michaelsauter]

While ODS uses Nexus, there should not be Nexus-specific things in the build tasks, as the configuration of build tools like Gradle does not depend on which artifact repository is used. In those places we should talk about "artifact repository", not "Nexus". The same applies to the ConfigMap and Secret.

There is however Nexus-specific implementation in the Go client, and this is required. However, we could create an interface and make the other code use that interface instead.

FYI @oalyman as suggested by you

[michaelsauter]

The start and finish tasks are tied to Nexus.

The build task extracted in #722 should be Nexus-agnostic, and those may choose to use a neutral name. However the central repo only works with Nexus right now, as it uses the Nexus API to discover artifacts.