Typos fix in db and periodic_task module
Typos fix in db and periodic_task module

Fix several typos in db/sqlalchemy/models.py and periodic_task.py

Change-Id: I7a4e72ee379f650192e656fe260d858fecea4637

eezhova/oslo.service,openstack/oslo.service,citrix-openstack-build/oslo.service,poznyakandrey/oslo.service