Set default imagePullSecret name when using NGINX Plus

[sjberman]

By default, our docs have users create the docker registry secret with the name nginx-plus-registry-secret. But we don't use this default in our actual installation. We force users to specify it.

We should use this default name in our installation to reduce the amount of fields that a user has to specify if they copy installation steps straight from the docs.

Acceptance

• If using NGINX Plus, the default imagePullSecret name is set to nginx-plus-registry-secret.
• Installation docs are updated to no longer include this field in the command examples.