Comment 3 for bug 991553

Jeroen is correct. sometime between 10.04 and 12.04 (i think) sudo started being more annoying about filtering environment variables. Note, though, you do no need the 'env' command, as sudo does it itself:

 sudo http_proxy=..... maas-import-isos

Additionally, you could actually just add:
  export http_proxy=http://example.com
to the /etc/maas/import_isos config file.