The MAAS commissioning page (http://{server-name-or-address}/MAAS/settings/) has, in the "Ubuntu" section, a drop-down list of Ubuntu versions. The trouble is that this list shows everything from 12.04 to 14.04, whether or not it's available. In fact, the default /etc/maas/bootresources.yaml file downloads only 12.04 and 14.04 images, so when using the default settings, the list in the Web interface will include non-functional options.
The same overly-broad list of boot options occurs in the node-specific pages obtained by clicking "Edit node" in the node summary screen.
$ dpkg -l '*maas*'|cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=====================================================-===================================================-============-===============================================================================
ii maas 1.5+bzr2252-0ubuntu1 all MAAS server all-in-one metapackage
ii maas-cli 1.5+bzr2252-0ubuntu1 all MAAS command line API tool
ii maas-cluster-controller 1.5+bzr2252-0ubuntu1 all MAAS server cluster controller
ii maas-common 1.5+bzr2252-0ubuntu1 all MAAS server common files
ii maas-dhcp 1.5+bzr2252-0ubuntu1 all MAAS DHCP server
ii maas-dns 1.5+bzr2252-0ubuntu1 all MAAS DNS server
ii maas-region-controller 1.5+bzr2252-0ubuntu1 all MAAS server complete region controller
ii maas-region-controller-min 1.5+bzr2252-0ubuntu1 all MAAS Server minimum region controller
ii python-django-maas 1.5+bzr2252-0ubuntu1 all MAAS server Django web framework
ii python-maas-client 1.5+bzr2252-0ubuntu1 all MAAS python API client
ii python-maas-provisioningserver 1.5+bzr2252-0ubuntu1
Good catch, thanks.