In the POSIX LDAP schema, there is no 'memberOf' attribute returned
in relation to which groups a person is a member of. Rather, it is
necessary to query the group objects which have the people as members.
This commit adds an additional filter, which if specified explicitly
searches for groups, rather than relying on the 'memberOf' attribute.
This enables Grafana to work with LDAP POSIX schema (e.g. OpenLDAP
etc.)

Signed-off-by: Alex Bligh <alex@alex.org.uk>