Allow skipping exercises.

- Catch a special exit signal 55 to notify that we want to skip an
  excercise.
- Move is_enabled_service to functions.
- Fix bug 928390.

Change-Id: Iebf7a6f30a0f305a2a70173fb6b988bc07e34292
diff --git a/exercise.sh b/exercise.sh
index 2072b23..15f264f 100755
--- a/exercise.sh
+++ b/exercise.sh
@@ -32,7 +32,10 @@
         echo Running $script
         echo "====================================================================="
         $EXERCISE_DIR/$script.sh
-        if [[ $? -ne 0 ]] ; then
+        exitcode=$?
+        if [[ $exitcode == 55 ]]; then
+            skips="$skips $script"
+        elif [[ $exitcode -ne 0 ]] ; then
             failures="$failures $script"
         else
             passes="$passes $script"