Add zuul_task_manager to expose control of tasks

There might be a time were we only want run the pre task, so rather
then copying the code into another modules, just allow the user to
override the task list that runs.

Change-Id: I97a40294ac3332bcce20ca4e3458c9988816c6c7
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -12,6 +12,13 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 ---
+# tasks/main.yaml
+zuul_task_manager:
+  - pre
+  - install
+  - config
+  - service
+
 # tasks/config.yaml
 zuul_user_name: zuul
 zuul_user_group: zuul