The Horizon Air Service Description (As of the time of this writing on 05/09/2016) describes the following allowance on utility servers:
"Horizon Air VMs are intended for use with desktop and terminal services applications. Special exemption is provided for customers who wish to use a VM instance as a utility server (s.a. domain controller, active directory server, DHCP relay or file server). 1 VM in the tenant may be used as a utility server without drawing from the desktop quota purchased. Any additional utility servers will count towards the desktop quota purchased by subtracting the total CPU and Memory resources consumed for utility service as expressed in terms of whole number of desktops from the total VMs purchased. "