This ticket is a continuation of issue #3973

The write_krb5info_file receives a linked list of struct fo_server
structures which contains the address, if already resolved, or at least
a host name in the struct server_common member structure. Since the
callback should already be synchronous and not do much work on its own, it
would be best if the callback was already invoked with the data provided,

There are two kinds of servers in the fail over module - primary and
backup. The backup servers are supposed to only be used temporarily
and sssd periodically tries to connect to one of the primary servers.
However, from the fail over code point of view, even adding a "backup"
server still means the server is added to the same linked list, just with
a flag denoting that the server is not primary, therfore iterating over
a single list would iterate over both the primary and backup servers.

Before changing the online callbacks, it would be useful to implement and
read the krb5_kdcinfo_lookahead option so that there is already an
upper limit when the callbacks write the extra host names.

The next step of implementation could be extending the online
callbacks that call the write_krb5info_file functions. There are
several of them, ad_resolve_callback, ipa_resolve_callback
and krb5_resolve_callback. The callbacks receive the current
struct fo_server instance. The callbacks would then keep iterating
over the linked list until either the list is exhausted or as many as
krb5_kdcinfo_lookahead items are processed. The host name from the
struct server_common structure would be read using fo_get_server_name
and written to the array passed to write_krb5info_file