OK, so we have a few things in-flight right now (e.g. jessie upgrades, T132450/HTTPS for carbon etc.), so it's about time to clean this all up.
This would be my suggestion:
[x] Upgrade install2001 to jessie, make sure our manifests work there (trivial)
[x] Deploy a new install1001 VM
[x] move install_server (DHCP, TFTP, preseed web) to install1001
[x ] move reprepro/apt (remember `/root/.gnupg/`!) to install1001
[ ] Remove all install_server-related roles out of carbon and designate it as a mirror server (T84817); reinstall carbon with jessie (T123733)
[x] (optionally) split the reprepro role out of install_server in puppet, they're really different roles
[x] Upgrade install2001 to match install1001 (i.e. add DHCP, point codfw's DHCP relays to install2001) (T84380)
[x] Set up apt on install2001 too (sync done with rsync in cron)
 use a reprepro hooks to sync apt across and possibly make apt.wikimedia.org HA (perhaps with gdnsd).
Any takers? @Dzahn maybe?
related patches: https://gerrit.wikimedia.org/r/#/q/topic:installserver+%28status:open+OR+status:merged%29