Improve ipcluster management

Refactor `distarray/tests/ipcluster.py` to take `start` and `stop` commands and change the  `setup_cluster` and `teardown_cluster` Makefile targets to use them.

`ipcluster stop` seems to be broken under Python 3, fix here:
https://github.com/ipython/ipython/commit/cb9fb2d98390465958418dd8c6540e9156f689db

But with that fix applied, this works.  Without that fix, this PR leaves things just as they were- `teardown_cluster` is broken under Python 3.