GEOS-5054, GEOS-5805 improvements for LDAPAuthenticationProvider

Adds some new options to LDAPAuthenticationProvider to ease connection with ActiveDirectory servers, but not only.
Options added are:
 * bindBeforeGroupSearch: permits roles extraction on servers not allowing anonymous searches
 * userFilter, userFormat: splits user binding and data extracting in distinct requests
 * adminGroup: allows administrator role remapping to a specific LDAP group