Add Server Spec for all_hosts Method

Update the Server spec to include testing the all_hosts method.
Update Server for all_hosts

Update the Server class to include the all_hosts method to return an
array of all currently monitored hosts.
Update Server for monitored_ Instead of all_

Update the Server class to change instance methods all_hosts and
all_hosts_services to monitored_hosts and monitored_hosts_services in
order to better provide context.