fix distro info failures by faking out the filename

this fixes https://bugs.launchpad.net/juju-core/+bug/1384175
Utopic test failures due to addition of vivid series

Instead of comparing the (dynamic) contents of /usr/share/distro-info/ubuntu.csv to a static list of series, how about we fake out the file being read and read something static?  Yeah, that sounds nice.
Why not const?