setup web-chi-03 full mirror in gnt-chi

as part of the load reductions efforts on gnt-fsn (#40100 (closed)), setup a new web mirror in the gnt-chi cluster.

this requires a working iSCSI setup because of the size of full mirrors (#40131 (closed)).

• iSCSI tests (#40131 (closed))
• VM setup
• puppet
• ldap
• nagios
• first mirror sync
• full mirror sync: in progress, ETA 2021-03-12 08:00UTC (28GB done out of 248GB in ~1h as of 2021-03-11 22:04UTC)
• benchmarks
• DNS update (in production!)
• hook into fastly

we're basically following (and making up as we go) this guide here: https://gitlab.torproject.org/tpo/tpa/team/-/wikis/howto/static-component/#web-mirror-setup