Adding two snapshot related task states

The first, 'image_pending_upload', indicates that the snapshot of a given
instance has been taken and it is being prepared for uploading to the
image service.

The second, 'image_uploading', indicates that the compute manager has
initiated upload to the image service.

Implements blueprint snapshot-task-states

Change-Id: I256c5d21a1d23b87d2060cca99eb9839c5b89161
Merge "Adding two snapshot related task states"