Updated test_ls_completed to accommodate new `.ssl` folder in sandbox.
Updated test_ls_completed to accommodate new `.ssl` folder in sandbox.

Also removed superfluous call to `dcos task ls` that was causing the
test to not actually test the listing of files for a completed task.

dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,mesosphere/dcos-cli,kensipe/dcos-cli,mesosphere/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,mesosphere/dcos-cli