cannot use fast path installer to deploy other than trusty

[Scott Moser]

$ tail /etc/maas/import_ephemerals
# The 14.04 release of MAAS can only enlist/commission using Ubuntu 14.04
# (Trusty Tahr) ephemeral boot images. The ephemeral images are quite large,
# much larger than the install images, so there is little point downloading
# them for older releases.
RELEASES="trusty"

That comment is quite informative, but the result of following it's advice is that you cannot install 12.04 via fast path installer.

This needs fixing. either in
a.) changing the comment to address that
b.) support installing 12.04 (or 13.10) with 14.04 ephemeral image (this generally "should work").
c.) changing the default