Please ensure it quotes activation keys to prevent a repeat of https://bugzilla.redhat.com/show_bug.cgi?id=1051580.

Here's a starting point, we need to be able to replace this snippet:
https://github.com/Katello/foreman-katello-engine/blob/master/app/views/unattended/snippets/_subscription_manager_registration.erb#L7

Don't forget, it goes to community-templates, then we'll just resync on the next Foreman release.

I ran into this today and FWIW here my findings (with Dominic's help):

with hosted RHN there's no possibility to use activation keys at the moment, so I use this for now:

subscription-manager register --username <%= @host.params['rhn_username'] %> --password <%= @host.params['rhn_password'] %> --name <%= @host.name %> --auto-attach

yum-config-manager --enable rhel-<%= @osver %>-server-optional-rpms

This got finished in sprint 19, but didn't get closed automatically for some reason. There is a final pending patch [1] that needs to get merged before I close this.

1. https://github.com/theforeman/community-templates/pull/54