Add elected info attribute

# What's This Do?

In monitoring Marathon I'd like to know if the appropriate number of leaders are elected. Therefore I need a way to determine if a host is a leader without relying on the comparison of hostnames in the `leader` attribute.

This PR adds an `elected` attribute to the `/v2/info` resource so that I can modify the Datadog plugin for Marathon to tell me the count of overall leaders! This follows the convention used in Mesos wherein the `/master/stats.json` has an 'elected' value of 1 or 0.

# Thanks

I appreciate you taking the time to look at this. If I can do anything to help a merge, let me know!