reusing container doesn't work

[robnagler]

When rsdockerspawner.get_object returns an container to dockerspawner start and remove is set, it expects the container to be removed, not still there, on login and then removes it:

20:34:58 [rsdockerspawner:532] slot_alloc: found slot=19 cname=/jupyter-robnagler pool=internal host=<redacted>
20:34:58 rsdockerspawner/rsdockerspawner.py:201:__docker_client
20:34:58 [dockerspawner:976] Removing container that should have been cleaned up: jupyter-robnagler (id: 7aafee3)
20:34:58 [dockerspawner:815] Removing container 7aafee3fcbed0065b18bcada63762eae93df00a744d4cba9b145a90f09c4e699

Fix with #14