Tests for nova plugin

@cyli just added a utility in af52bdf8de98a9324128eaa6e708ce2529a68574 for this, [`json_request`](https://github.com/rackerlabs/mimic/commit/af52bdf8de98a9324128eaa6e708ce2529a68574#diff-a82661428a31e375ffd0d089731c1077R143).