On ubuntu contents of /var/run do not persist between reboots. Devstack
uses /var/run/uwsgi as home for wsgi sockets. This means that after
rebooting the machine services, that rely on uwsgi would fail to start.
Currently it affects keystone.service and placement-api.service.
This patch changes delegates directory creation to systemd-tmpfiles,
which would run on startup.
Change-Id: I27d168cea93698739ef08ac76c828695a49176c7
Closes-Bug: #1692767