Handle Erroneous Cron Strings in TimerDriver

This change introduces error-handling in case of invalid numeric
values passed to APScheduler's CronTrigger, which fires an unhandeled
ValueError in such a case. Triggers with invalid values will get
ignored, just like tiggers with an invalid 'cron string' (see line 65 of
the same file).

Change-Id: I4fd5d80c0fb3a4fe27a1687c607774eeed3f9db6

tests/fixtures/config/broken/git/common-config/zuul.yaml

@@ -29,3 +29,10 @@
     name: org/project2
     check:
       jobs: []
+
+- pipeline:
+      name: broken_periodic
+      manager: independent
+      trigger:
+        timer:
+          time: "*/30 * * * * 300"

zuul/driver/timer/__init__.py

@@ -73,8 +73,19 @@ class TimerDriver(Driver, TriggerInterface):
                         second = parts[5]
                     else:
                         second = None
-                    trigger = CronTrigger(day=dom, day_of_week=dow, hour=hour,
-                                          minute=minute, second=second)
+
+                    try:
+                        trigger = CronTrigger(day=dom, day_of_week=dow,
+                                              hour=hour, minute=minute,
+                                              second=second)
+                    except ValueError:
+                        self.log.exception(
+                            "Unable to create CronTrigger "
+                            "for value '%s' defined in "
+                            "pipeline %s",
+                            timespec,
+                            pipeline.name)
+                        continue
 
                     job = self.apsched.add_job(
                         self._onTrigger, trigger=trigger,