Example use cases: * Container is expected/intended to be impure (results differ from one run to the next) * Test/diagnostics container The [[Containers API]] specifies a "filters" parameter which (when implemented) could be used to achieve this with something like @filters=[[created_at,=,now]]@. Presence of the "API" runtime constraint should also automatically skip reuse. Implementation: * Add a 'reuse' flag, which defaults to true. When set false a new container should always be created. * Add '"reuse": false, ' to the 'Hello, Crunch' example at http://doc.arvados.org/install/crunch2-slurm/install-test.html As for the future "filters" parameter, if/when that gets implemented, the 'reuse' flag should take precedence if set to false.