Snapshot worker references non-existant exception.

This patch removes a reference to the exception Unauthenticated that no longer
exists in novaclient. Removing of this code should not change any behavior
except fixing job fault messages to include the real error.

Fixes issue 260
Merge pull request #261 from ameade/remove-unauthenticated-exc

Snapshot worker references non-existant exception.
Fix issue where 'image' variable is not defined

Fixes issue 262
Small Refactor in process_job function
Only set snapshot jobs to DONE if retention has been processed

Fixes issue 264
Merge pull request #263 from ameade/fix-image-variable-undefined

Fix issue where 'image' variable is not defined
Get next job based on updated_at timestamp.
Minor refactor in snapshot worker
Merge pull request #265 from ameade/change-process-job

Only set snapshot jobs to DONE if retention has been processed
Merge pull request #267 from esheffield/get-job-by-updated

Get next job based on updated_at timestamp.
Add expected update_job mox call to allow test to pass.
Merge pull request #269 from rackerlabs/unittest-mox-fix

Add expected update_job mox call to allow test to pass.
Fix broken unit test.
Merge pull request #270 from esheffield/fix_unit_test

Fix broken unit test.
Reverts the previous fix to unit test

Two fixes were merged to fix rm3453. This commit reverts one of those.
Merge pull request #271 from rackerlabs/rm3453

Reverts the previous fix to unit test