Removed images from 2013.
Fixed indentation inside map.
Added all available public VM images, skipping those from 2013.
Update Python test to iterate over all available images rather than a sample.
Updated test data to latest images since we removed VM images from 2013.
Added support for pseudo-images to easily refer to latest VM image versions.
Factored out common URL string formatting into a separate method.
Use '/bin/echo' rather than 'echo' to avoid using shell builtins.

/bin/echo supports -n, whereas on OS X, 'echo -n ...' shows '-n'
in the output.
Added testing for golden input/output tests.
Moved config tests from vm_images_test.py to config_test.py .
Moved common URL computation from config.py into common.py .

This will allow for reuse with other config handlers.
Added some tests to verify that this refactoring was a no-op.
Moved reading of startup-script referenced files '%file:<...>' to common.

Also added a test to document (and verify) before/after behavior.
Factored out IsUrl() from config into common, simplifying repeated code.
Added diskType -> URL and image -> URL functions.