connector/ldap: fix case where groups are listed on the user entity

Support schemas that determine membership by having fields on the
user entity, instead of listing users on a groups entity. E.g. the
following schema is now supported when it wasn't previously:

    cn=eric,cn=user,dn=exapmle,dn=com
    objectClass=myPerson
    cn: eric
    uid: eric
    email: eric@example.com
    memberOf: foo
    memberOf: bar

    cn=foo,cn=group,dn=exapmle,dn=com
    objectClass=myGroup
    cn: foo

    cn=bar,cn=group,dn=exapmle,dn=com
    objectClass=myGroup
    cn: bar