Autocluster all the things!

Significant Changes:

• Renamed from rabbitmq-autocluster-consul to rabbitmq-autocluster
• Add support for etcd2 and DNS based discovery
• Refactor to be a more generic structure for pluggable backends
• Node type (disc, ram) is now configurable via environment variable or setting

Behavioral fixes:

• Nodes will no longer try and join a cluster they are already a member of
• Nodes will no longer try and join nodes in a cluster provided by the discovery service if they are not available
• Consul TTL health checks now update Consul after half the TTL duration, ensuring nodes are not arbitrarily removed from discovery to only be added right back in
• If the discovery service is not available, the node will start up in stand-alone mode