Looks like maas-enlist-udeb (the maas-enlist function for d-i, which gets run when one selects the ubuntu-server ISO menu option "Multiple server install using MAAS", has been failing at least since Trusty.
Attached are screenshots and logs for attempting a provisioning via MAAS, but the enlist step fails: curl is not found, even though the curl-udeb package is installed.
The system would normally silently fail to enlist and logs be somewhat hard to get since maas-enlist-udeb.postinst forces a shutdown at its end, regardless of the result of running /usr/bin/maas-enlist. The postinst file was modified to remove the shutdown in order to get the logs.
Sep 4 13:09:28 netcfg[11509]: DEBUG: Network config complete
Sep 4 13:09:28 netcfg[11509]: DEBUG: No interface given; clearing /etc/network/interfaces
Sep 4 13:09:28 netcfg[11509]: DEBUG: Writing informative header
Sep 4 13:09:28 netcfg[11509]: DEBUG: Success!
Sep 4 13:09:28 netcfg[11509]: DEBUG: Writing loopback interface
Sep 4 13:09:28 netcfg[11509]: DEBUG: Success!
Sep 4 13:09:28 netcfg[11509]: DEBUG: Writing DHCP stanza for ens3
Sep 4 13:09:28 netcfg[11509]: DEBUG: Success!
Sep 4 13:09:29 main-menu[224]: DEBUG: resolver (libavahi-common3): package doesn't exist (ignored)
Sep 4 13:09:29 main-menu[224]: INFO: Menu item 'maas-enlist-udeb' selected
Sep 4 13:10:05 main-menu[224]: (process:11620): /usr/bin/maas-enlist: line 215: curl: not found
Sep 4 13:10:05 main-menu[224]: DEBUG: resolver (libavahi-common3): package doesn't exist (ignored)
Sep 4 13:10:05 main-menu[224]: INFO: Menu item 'user-setup-udeb' selected
Sep 4 13:10:08 main-menu[224]: INFO: Menu item 'user-setup-udeb' succeeded but requested to be left unconfigured.
Sep 4 13:10:09 main-menu[224]: DEBUG: resolver (libavahi-common3): package doesn't exist (ignored)
Sep 4 13:10:14 main-menu[224]: INFO: Menu item 'save-logs' selected
As you can see, the curl package is installed; but as logs show the binary is not available.