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 should be backported
to stable/havana for this reason.

Change-Id: Ib5afb321cef2b7ad74e69a3fd0d1dad469f78b11

lib/swift

@@ -687,6 +687,11 @@ function stop_swift {
         swift-init --run-dir=${SWIFT_DATA_DIR}/run all stop || true
     fi
     # Dump all of the servers
+    # Maintain the iteration as screen_stop() has some desirable side-effects
+    for type in proxy object container account; do
+        screen_stop s-${type}
+    done
+    # Blast out any stragglers
     pkill -f swift-
 }