Expose more information about the cluster via the HTTP API and UI

There are some configuration options in Consul that would be useful for operators but are not exposed once the agent is running.

Some of these include:
- Primary datacenter
- Whether ACLs are enabled on all agents
- Whether Connect is enabled on all servers
- Whether RPCs are encrypted with TLS

Since the information for most these is distributed around the cluster, we would need to decide what's a good way to display the information.

For example, if ACLs are enabled in all but a few agents, we should likely provide a way to find out what agents do not have them enabled.

Start by tracing the existing HTTP API and UI entry points for cluster and agent configuration. Determine how primary datacenter, ACLs, Connect, and RPC TLS status are distributed across the cluster, then define a consistent way to expose partial or differing values and how operators can identify exceptions.

Written by the indexing model from the issue text.