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.
Merge remote-tracking branch 'remotes/origin/ldap-multi-server' into ldap-multi-server