devstack/plugin.sh: use screen_stop
devstack/plugin.sh: use screen_stop

A custom, direct screen call is being used to stop inspector
processes.  This breaks grenade testing for me as no screen is used in
that environment.

This patch suggests adopting screen_stop instead.  See also project
devstack/functions-common:screen_stop

Change-Id: I63d0c7400ecdff4333ffc31b6607d94f8cf20e18

openstack/ironic-inspector,openstack/ironic-inspector