Switch the LXD compute driver test to non-voting
LXD 2.0.11 grew a new feature ('description' tag for a container)
and this has been added to the stable Ubuntu Xenial 16.04 release.
Unfortunately, pylxd 2.2.2 can't handle this extra attribute and
crashes out. This was fixed in pylxd 2.2.4, but has not been ported
to the 2.0.x series.
As such, we switch the test job to non-voting until an upstream
fix is available.