Enter Nexus credentials on CustomersDot staging & production

CustomersDot (https://customers.gitlab.com/) will send leads to the Nexus API.

We need to get Nexus credentials into the config/secrets.yml using environment variables:

  nexus_url: "<%= ENV['NEXUS_URL'] %>"
  nexus_client_id: "<%= ENV['NEXUS_CLIENT_ID'] %>"
  nexus_client_secret: "<%= ENV['NEXUS_CLIENT_SECRET'] %>"

CustomersDot

Env variable	staging	production
NEXUS_URL	https://staging.ci.nexus.gitlabenvironment.cloud	https://production.ci.nexus.gitlabenvironment.cloud/
NEXUS_CLIENT_ID	To be provided by @dparker
NEXUS_CLIENT_SECRET	To be provided by @dparker