Update pylxd to 2.2.4 upper constraint as fix for lxd 2.0.11
This overrides the upper-constraint for pylxd as a temporary
measure to allow the nova LXD compute driver to work properly
with recent updates to LXD.
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.