Filter for health Passing

[jhmartin]

Please consider adding a option to request just 'passing' healthcheck nodes from the Health api. This is done by appending ?passing to the query. I can work around this by running

{% set nodes = ['%s:%s' % (r['Node']['Address'], r['Service']['Port']) for r in consul.health.service('httpd?passing')] %}

However, it would be much cleaner if the API allowed for something like

consul.health.service('httpd').passing