[Authentication] realmd: Faster discovery, generic kerberos discovery

[Stef Walter]

Some more patches for realmd.

The first patch is one that makes the discovery of kerberos realms much
faster:

https://bugs.freedesktop.org/show_bug.cgi?id=53956

In particular discovery of IPA is harder to do in a fixed amount of
time. We actually try to connect to the server to retrieve its
certificate, and this can block for a long time given a firewall. This
was slowing down discovery of non IPA domains.

So the patches at the above bug, integrate the discovery into a single
class, which discovers everything about a server all at once. It does
IPA server in parallel and limits initial connection attempt to 5
seconds. If server is discovered as being AD, then we short-circuit IPA
discovery.

Also another patch, which enables discovery of generic kerberos realms:

https://bugs.freedesktop.org/show_bug.cgi?id=53958

We can't actually enroll in generic kerberos realms, but discovery is
nice to have. gnome-online-accounts wants to use this feature.

Any review, sanity checking, or testing is super appreciated.

Cheers,

Stef