service: allow multiple names and _srv_ ad_server option
realmd checks if the 'ad_server' option is set in sssd.conf before calling adcli to remove the host from the AD server. If set the value is used as value for dcli's '--domain-controller' option. But if multiple names are set in sssd.conf this currently fails because the whole string is used.
With this patch the 'ad_server' option is properly evaluated and only the first domain controller name is used.