Add new notifications types for volumes/snapshots

Now we have only two types of notifications about volumes/snapshots:
volume/snapshot existence and their size.
This blueprint is about collecting and viewing notifications of different types.
We need to get information about which events have occurred: volume/snapshot
has been created or deleted or updated(renamed or modified description),
volume has been resized or attached/detached. This will allow to process
additional events and will improve the overall Ceilometer functionality.