Refactor key cleanup on client.
Remove `del foo` in favor of `context.cleanup_keys()`
Remove IpclusterTestCase

This was only providing a self.client attribute which was only actually
used in one test class. Its checks and cleanups were unnecessary.
Remove __del__ def from Context
Change cleanup_keys to cleanup.

This reflects the action rather than the implementation since keys are
an implementation detail.
Fix purge_cluster script
Cleanup context in tests. Add Client.close() to Context.cleanup().
Merge pull request #306 from cowlicks/cleanup-test

Cleaning up tests, removes exceptions.