fix: support users and groups in distinct DN bases

[ivan-gomes]

resolves #194

The important changes are:

• use of userData.getDn() instead of userAttributesUsername + '=' + username to get the entire DN, e.g. if ldap.user.dn.pattern=uid={0},ou=users this will resolve to uid=billy,ou=users instead of just uid=billy
• provide groupSearchBase for the group LDAP search

This change should be fully backwards compatible, i.e. should not require any configuration change for currently functioning deployments. It will add support for LDAP structures (currently incompatible) as described in #194 using a configuration like the following:

ldap.provider.base=dc=example,dc=com
ldap.provider.url=ldap://ldap.example.com
ldap.user.dn.pattern=uid={0},ou=users
ldap.group.search.base=ou=groups

[HuiJun]

I'm an old man, and I like the faster performing 'for', but everything else seems good to me.