move realm_to_serverid to installutils module

To avoid cyclic imports realm_to_serverid function had to be moved to
installutils from dsinstance.

Required for: https://fedorahosted.org/freeipa/ticket/4925

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipaserver/install/installutils.py

@@ -1105,3 +1105,6 @@ def check_version():
             )
     else:
         raise UpgradeMissingVersionError("no data_version stored")
+
+def realm_to_serverid(realm_name):
+    return "-".join(realm_name.split("."))