trigger: suppress cycle warnings for root triggers #7122
Description
When you trigger a task, you may get a warning if the task is not valid at the specified cycle point.
E.G, If I tried to trigger 20191209T0900Z/foo, I may get the warning:
WARNING - Invalid cycle point for task: foo, 20191209T0900Z
This is useful. However, If I try to trigger a cycle point ^ which is implicitly ^/root, I may get this warning if there are tasks defined within the workflow which are not valid for this particular point, e.g:
INFO - Command "force_trigger_tasks" received. ID=5e0d6570-7a56-428c-97fe-2d30542b7082
force_trigger_tasks(flow=[], flow_wait=False, on_resume=False, tasks=['^'])
WARNING - Invalid cycle point for task: foo, 20191209T0900Z
WARNING - Invalid cycle point for task: wipe_bar, 20191209T0900Z
WARNING - Invalid cycle point for task: fool, 20191209T0900Z
This is not useful.