Add consistency support to Health endpoints

[ciscopony]

This is a "To-Do" in the current code, I've tested adding:

def ENDPOINT(self, ...
consistency=None):
...
consistency = consistency or self.agent.consistency
if consistency in ('consistent', 'stale'):
params.append((consistency, '1'))
...

To the ENDPOINT=service/checks/state/node blocks and it appears to work, I'd be happy to submit a PR. Having this functionality would help with a performance issue we hit occasionally during cluster leadership changes.