ldap group mapping support for active directory

We have been trying to use the new LDAP/Roles Mapping feature from the current archiva 2.0.0-SNAPSHOT. Unfortunately this didn't work with our active directory because of the hardcoded `uniquemember` attribute and the way the query for groups is constructed.
These changes introduce the new option `ldap.config.groups.member` to configure the group member attribute.
Also a user is now considered part of a group if the group either contains the username (previous behaviour) or the full dn of the user (which is the case in our active directory).