Adding finer notifications.

This commit adds notifications which offer a finer view of what's going on
    inside Glance.

New Info Notifications (v1, v2)
image.create: Emitted when the image is first saved/reserved.
image.prepare: Emitted right before Glance begins uploading to it's store.
image.activate: Emitted when an image's status is set to active.

New Error Notifiactions (v2)
image.upload: Emitted on StorageFull or StorageWriteDenied exceptions to
                  mimic v1

Partially implements: bp:glance-notifications-deux

Change-Id: I2a4a48f6b57ab4739472f22eb42f9307266e6072
Clean dangling image fragments in filesystem store

Fixes bug LP 1104924

Previously when a PUT or POST of image content was terminated
prematurely by the client, the partially saved image file was
left behind in the data directory.

Change-Id: Id601816735e4138cd7623dad4d90be67448292c8
Merge "Clean dangling image fragments in filesystem store"
Merge "Adding finer notifications."