Add endpoint for operational monitoring of server healt status

As discussed in mailing list, we should add some endpoint which allows monitoring of server's health status. Use case is for example general health check from loadbalancer to know which server can be used to handle requests, or more detailed health checks from operational monitoring systems like Nagios which alerts sysadmins when something goes wrong and allow them to perform quick action to correct problem. This detailed info should cover areas like database connection, connection to remote systems from user federation providers (LDAP, Kerberos server, custom REST based providers...), correct work of social identity providers (google, twitter) etc.

We could have a single health http endpoint without authentication that returns a 200 or 500 matching general status and also returns a json doc with details. Something like:

{
  realm: ok,
  users: ok,
  realmCache: error,
  userCache: error,
  identity-providers: {
    google: ok,
    facebook: error
  },
  user-federation: {
    ldap: ok
  }
}

BTW we should think about which detailed fails should mean general fail. For example fail of social identity-provider like google should not mean general fail as KC itself can be still used for login!