Fixes task_log_get and task_log_get_all signatures

Fixes signatures for the above methods in nova/db/api.py
based on the recent TaskLog refactor on sqlalchemy/api.py.
Also adds unit tests for TaskLog.

Fixes Bug 1102652.

Change-Id: Id7ce74b8941d9eb6a50dfbfaa62e0fe05dd467c2
Merge "Fixes task_log_get and task_log_get_all signatures"