Create a romana health plugin

[jbrendel]

This plugin should use information available in our KV store to inform vpc-router about the health and eligibility of cluster nodes.

Most likely, this plugin may just focus on reporting cordoned off nodes as 'questionable', so that vpc-router can initiate the route failover before this node actually fails.

The desire for this feature was reported as issue 26 against the vpc-router: romana/vpc-router#26

However, the Romana plugin is the better location for it.