Provide latest ACL index on an unprivileged API endpoint

### `consul version` for both Client and Server
Server: 0.7.2, present in current version based on docs

### Description of the Issue (and unexpected/desired result)

When running ACL replication the /acl/replication endpoint in the DCs replicating the master return the latest index they have replicated. However, this index is only available on the master at /acl/list, which is a privileged endpoint that returns sensitive data. Furthermore, the /acl/list endpoint returns far more data than is needed to get the current master index, and it is required that the system consuming the endpoint iterate over a list to find the highest index.

I'd like to have my monitoring check my non-master DCs for lag, however I do not want the monitoring system to access a management token for security reasons. Furthermore, I'd prefer my monitoring check not have to parse all the ACLs in the system to get the current master index, as I foresee potential performance issues with that as my list of ACLs grow.

Please create an acl endpoint that simply returns the current ACL index and which does not require a privileged token. As the /acl/replication endpoint doesn't require a token, and as the index isn't privileged information, no auth would be preferred. This will be very useful for monitoring ACL replication with monitoring systems. Thanks.