The active directory user principal name format typically takes the form user@domain, which is what
the current implementation expects. However, active directory also allows the definition of other
suffixes that are not actual domains. A user can still authenticate using this user principal name
but the behavior of our realm would cause it to fail as it parsed the suffix as a domain and used it
as the search base for the user. Instead, we should use the default user search base and only look
for entries that have this exact user principal name. In a scenario where a realm is configured for
multiple domains in the same forest, the search base should be the base for the entire forest.
relates elastic/x-pack-elasticsearch#1744
Original commit: elastic/x-pack-elasticsearch@de00c4817e
6fdad6039f