Add LDAP support for broadening search context

[ghsolomon]

Our current xhLdapConfig requires that a baseUserDn and baseGroupDn be specified per server. However, in several client applications, we connect to a single LDAP server and need to query multiple OU's (i.e. "US", "UK" and "Groups"). Currently, the only way to do this is by listing duplicate servers in config for each OU we wish to search.

Ideally, we should provide a better mechanism for specifying a broad search context that lists multiple OU's without needing to repeatedly list the same server host.