Move task_log functions to conductor

This moves the use of the following functions in compute/utils to
the conductor:

  task_log_get()
  task_log_begin_task()
  task_log_end_task()

This removes the db import from compute/utils.py

Related to blueprint no-db-compute

Change-Id: I6ed0563969afee78d9f2970308bc7f91e5690159
Merge "Move task_log functions to conductor"