* postinst: write /etc/apt/sources.list.d/alt-nodejs-els-rollout.list with the els-alt-1..6 gradual-rollout slots on rollout.alt.tuxcare.com (active) and their slot-N-bypass counterparts (commented out), so the ALT slot rollout reaches clients gradually instead of only at graduation to stable. The rollout host has no channel segment and takes no credentials, so no /etc/apt/auth.conf.d entry is added. postrm removes the file. (ELS-2641)