Add is_ubuntu function

This replaces all of the [[ "$os_PACKAGE" = "deb" ]] tests, except when
those tests are before straight calls to dpkg.

Change-Id: I8a3ebf1b1bc5a55d736f9258d5ba1d24dabf04ea
diff --git a/unstack.sh b/unstack.sh
index 20ba17b..81ce088 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -65,7 +65,7 @@
         # If tgt driver isn't running this won't work obviously
         # So check the response and restart if need be
         echo "tgtd seems to be in a bad state, restarting..."
-        if [[ "$os_PACKAGE" = "deb" ]]; then
+        if is_ubuntu; then
             restart_service tgt
         else
             restart_service tgtd
@@ -85,7 +85,7 @@
         sudo rm -rf $CINDER_STATE_PATH/volumes/*
     fi
 
-    if [[ "$os_PACKAGE" = "deb" ]]; then
+    if is_ubuntu; then
         stop_service tgt
     else
         stop_service tgtd