[BUG] Task will choose the wrong template when updating its first-used template

Bug re-produce method.
1.Use template1 to create task task1
2.Use tempate2 to re-define task task1
3.Update template1
task1 will be updated using template1.

Start by reproducing the three listed steps in Kapacitor: create a task with template1, redefine it with template2, then update template1. Trace the task/template update path and verify that task1 remains associated with template2 rather than being updated from template1.

Written by the indexing model from the issue text.