The SRU for Ubuntu bug #1479496 (upstream bug #1422736) backported commit 4b6ed760d4303744907feefd81e60f38ae3750ef to vivid, but upstream bug #1431449 fixed a regression in that commit, and the regression fix wasn't backported.
So vivid's python-neutronclient 1:2.3.11-0ubuntu1.1 is affected by bug #1431449, causing it to blow up when endpoint_url is not specified.
The SRU for Ubuntu bug #1479496 (upstream bug #1422736) backported commit 4b6ed760d430374 4907feefd81e60f 38ae3750ef to vivid, but upstream bug #1431449 fixed a regression in that commit, and the regression fix wasn't backported.
So vivid's python- neutronclient 1:2.3.11-0ubuntu1.1 is affected by bug #1431449, causing it to blow up when endpoint_url is not specified.