fix(controller): ignore KeyError when purging user from etcd

Fixes #1239.

TESTING: Run `docker logs -f deis-controller` to watch for errors (since this happens in a Django signal handler, the client doesn't see it.) Create a user with `deis register`, cancel the user with `deis auth:cancel` without uploading a key, then re-register the same username. Ensure everything worked and that no errors appeared in the controller log.

We have a controller unit test that covers this exactly, but since it's isolated it stops short of interacting with the builder, and therefore succeeds. We will add an integration/regression test to #1237 before we let it be merged.