HUE-2012 [core] AD multidomain support

ldap_servers takes precidence over normal ldap config.
backward compatible since original ldap config can be provided.
Add server selection logic on front end.
Split tests by configs: test_ldap_deprecated and test_ldap.
The former will test deprecated configs and the later will test new configs.
The Synchronization middleware will choose the first LDAP Server
config when deciding which server to synchronize with.