Avoid stuck task_state on snapshot image failure

Fixes bug LP 1101136

Previously if the glance interaction failed prior to an
instance being snapshot'd or backed up, the task state
remained stuck at image_snapshot/backup.

The normal task state reversion logic did not kick in,
as this is limited to the compute layer, whereas the
intial glance interaction occurs within the API layer.

Now, we avoid this problem by delaying setting the task
state until the initial image creation has completed.

Change-Id: Id498ae6b3674306743013e4fe99837da8e2031b5
Merge "Avoid stuck task_state on snapshot image failure" into stable/folsom