nexus3

Describe the bug
The created ingress resource is annotated with: "kubernetes.io/ingress.class: nginx".
I use traefik, not nginx. The nexus-operator is unusable with a non-nginx ingress.

To Reproduce
Steps to reproduce the behavior:

1. Create an nexus instance with networking.exposeAs: Ingress
2. See that the ingress is created with a annotation: "kubernetes.io/ingress.class: nginx"

Thinking off the top of my head:

1. On first run, do something like docker run --rm -it -v $(pwd):/root nexus3-cli login
2. Change the login command to write two files:
   1. The usual ~/.nexus3-cli
   2. An additional ~/.nexus3-cli.env
3. On subsequent uses, use something like docker run --rm -it --env-file ~/.nexus3-cli.env CMD ...
4. Suggest an alias to be added to the user's pr