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
Change method name to test_get_volume_stats

Change test method name from test_update_volume_stats2 to
test_get_volume_stats in cinder/tests/test_eqlx.py.

It was pointed out by Change-Id:
I0642bdb911ca72517ed655f795e0055f4c4654b8

Change-Id: Id21c67e3e1151573e69ef21714fbf493f62ad425
Merge "Add unit tests for volume reserve and unreserve"
Merge "Add attach/detach notifications"
Merge "Change method name to test_get_volume_stats"