Add attach/detach notifications

Add the following notifications:
  - attach.start
  - attach.end
  - detach.start
  - detach.end

Blueprint: attachment-notifications
Change-Id: I4a4b123468c139bc2bcd948a1e7438c3830b5e42
Add unit tests for volume reserve and unreserve

This adds unit tests for successful reserving and unreserving, and vice
versa for not being in the right status.

Change-Id: Iaa777055315251efa4a14976e25b9baab222f934
Merge "Add unit tests for volume reserve and unreserve"
Merge "Add attach/detach notifications"