snf_django: Handle invalid filter expression

Handle case of invalid expression in the '--filter-by' option of
management commands by raising a 'CommandError' with a descriptive
exception message.
cyclades: Fix snapshot status in 'volume' views

Map the internal status of the image in Plankton in the status that is
specified in the blockstorage API.
burnin: Provide tests for snapshots

Extend burnin testing suite to provide snapshots functionality.

Closes #265
burnin: Delete temporary containers

Use kamaki's 'del_container' function with "delimiter='/'" in order to
delete temporary containers and all their contents.