Backport 'Make stop_swift() more robust for Grenade'

stop_swift() wasn't calling screen_stop() so the pid files and screen
sessions were not being cleaned up.  DevStack doesn't really care but
Grenade does for the 'base' copy of DevStack.  This is backported
to stable/havana for this reason.

Change-Id: Ib5afb321cef2b7ad74e69a3fd0d1dad469f78b11
Merge "Backport 'Make stop_swift() more robust for Grenade'" into stable/havana