Use class attribute to represent 'project'

When sending a notification a resource_type is required, such as a
'project'. This value is consistent between the notification.created,
updated, delete, disabled methods in the Manager class, so use a class
attribute to represent it.

Change-Id: I65cc0ffb1d8811ae9f518492f998e426005345f0
Merge "Use class attribute to represent 'project'"