NovaBase.delete() rename to NovaBase.soft_delete()

Name of this method is misleading. This method actually does not delete entry
from DB instead of that it only marks entry as deleted as well as
query.soft_delete(). Furthermore we have query.delete() that deletes from DB
all matched entries. So it is very important to rename this method to avoid
misunderstanding.

blueprint db-session-cleanup

Change-Id: Ic8b92ce1c087794aca4faafa9c68c72d444e5f9e
Merge "NovaBase.delete() rename to NovaBase.soft_delete()"