This commit switches TimeoutStopSec in DevStack's systemd unit files
from "infinity" to "300". There are two motivations for that change:

* 5 minutes should be more than enough to stop a service.
* systemd included in CentOS 7 and RHEL 7 doesn't support "infinity" as
a value, "0" should be provided instead. When "infinity" is set,
systemd will kill the service instantly, leaving service children
processes orphaned. Instead of differentiating here, we can just set a
sane, finite number.

Closes-Bug: 1731275
Change-Id: I0a079ea9879fa4fbba23104c2f5ab6e0721a2a2a