Allow notification settings in variables rather than the json file sd_notifications.json (remains as a fallback)

Add suggestions from PR#5

+readability

Make more idiomatic

fixed variable templating/interpolation for host_vars

* Some whitespace cleanup
* Some 'not x in y' to 'x not in y'
* Switch 'dict.has_key(key)' to 'key in dict'
* Change from try/except on error lookup to error.get()

catch KeyErrors in error formatter

example of `content` that throws KeyError
```
content = {u'errors': [{u'subject': u'token', u'type': u'invalid_token'}],
 u'message': u'Invalid token'}
```