Deny terminate task as parent, do not underflow priority 1
Deny terminate task as parent, do not underflow priority 1

- a terminate task cannot be a parent task by definition
- a child task will use the best priority between the current request and the
  parent's - 1. If the parent was 1, it would underflow.
- a task could technically trigger a terminate task, so if the request's
  priority is 0, it must not be changed.

R=iannucci@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2938143002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py