Provide finite value for systemd's TimeoutStopSec
Provide finite value for systemd's TimeoutStopSec

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

nawawi/openstack,nawawi/openstack,noironetworks/devstack,noironetworks/devstack,group-policy/devstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,group-policy/devstack,openstack-dev/devstack