FO: Add an API to reset all servers in a single service
    
    Required for:
        https://fedorahosted.org/sssd/ticket/2639
    
    Previously, we had a function that allowed the caller to reset the
    status of all services in the global fail over context. This patch adds
    a new function that allows the caller to reset a single service instead.
    
    The main user would be IPA subdomain provider that might need to reset
    the status of an AD trusted domain on demand.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>